Initial support for using Sonar in Swift projects (#106)

Summary:
This PR adds support for using SonarKit clients in Swift apps. Fixes #13, fixes #87

1. Swift can't import Obj-C modules which have C++ headers. For this reason, we use SonarKit as an Obj-C++ wrapper around Sonar, which is written in C++. Due to search path misconfiguration, trying to import SonarKit into a Swift project would import `xplat/Sonar/SonarPlugin.h` instead of `iOS/SonarKit/SonarPlugin.h`, which caused `file not found` errors for C++ stdlib imports like #28 because new projects don't have their search paths set up correctly.
2. The network and layout plugins have C++ definitions (struct methods, classes) in some of their headers. This causes the compiler to get confused for Swift projects, because it only supports importing Objective-C files in umbrella headers, meaning that the `SonarKit` won't build.

1. I updated the `HEADER_SEARCH_PATHS` of SonarKit.podspec's build configuration to include `${PODS_ROOT}/Headers/Private/SonarKit/**` first, which alleviates the search path issue. The Obj-C `Sample` project seems to have worked around this by including a hardcoded `${PODS_ROOT}/SonarKit/**` search path in the pbxproj, which is why Sample works but new projects (like those referenced in #28) don't. I removed this since it's no longer necessary.
2. I added a `SampleSwift` app to demonstrate using Sonar with a Swift project.
3. Because the Podfiles for `Sample` and `SampleSwift` referenced podspecs using `:podspec` instead of a concrete version, Cocoapods wouldn't copy local header files (instead, it downloads them from the source). To enable local development of these sample apps using `:path`, I added a symlink to SonarKit.podspec in the root of the directory.
4. I changed SonarKit.podspec to use a tag-based `source`, since v0.0.1 pulls from the master branch of this repo.

The layout and network plugins still don't work with Swift - in order to fix this, we'll need to work on extracting the C++ out of their headers and writing Obj-C++ wrappers for them. I decided to push this off to a later PR since this one is quite large already.

This means that we need to be able to `import SonarKit` without importing all the network/layout plugin headers. In order to make this work, I made "SonarKit/Core" the spec's `default_subspecs`.

priteshrnandgaonkar, let me know if you have any thoughts on this implementation. You can verify that the SampleSwift app works by checking out this branch, `pod install`ing in the SampleSwift directory, and building it :)

![image](https://user-images.githubusercontent.com/1168853/41928625-ac195bd8-792a-11e8-82b8-65d6233a1fbb.png)
Pull Request resolved: https://github.com/facebook/Sonar/pull/106

Reviewed By: jknoxville

Differential Revision: D8890010

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 449305bcc5cbeb5787c23f51b1ecb80a5cbdad32

iOS/Podfile.lock

@@ -1,58 +0,0 @@
-PODS:
-  - boost-for-react-native (1.63.0)
-  - CocoaAsyncSocket (7.6.3)
-  - CocoaLibEvent (1.0.0)
-  - DoubleConversion (1.1.5)
-  - Folly (1.0.0):
-    - boost-for-react-native
-    - CocoaLibEvent (~> 1.0)
-    - DoubleConversion
-    - glog
-    - OpenSSL-Static (= 1.0.2.c1)
-  - glog (0.3.4)
-  - OpenSSL-Static (1.0.2.c1)
-  - PeerTalk (0.0.2)
-  - RSocket (0.10.0):
-    - Folly
-  - Sonar (0.0.1):
-    - Folly
-    - RSocket
-
-DEPENDENCIES:
-  - CocoaAsyncSocket (~> 7.6)
-  - DoubleConversion (~> 1.1)
-  - Folly (~> 1.0)
-  - glog (~> 0.3)
-  - PeerTalk (~> 0.0)
-  - RSocket (~> 0.10)
-  - Sonar (~> 0.0.1)
-
-SPEC REPOS:
-  https://github.com/cocoapods/specs.git:
-    - boost-for-react-native
-    - CocoaAsyncSocket
-    - CocoaLibEvent
-    - DoubleConversion
-    - glog
-    - OpenSSL-Static
-  https://github.com/facebook/Sonar.git:
-    - Folly
-    - PeerTalk
-    - RSocket
-    - Sonar
-
-SPEC CHECKSUMS:
-  boost-for-react-native: 39c7adb57c4e60d6c5479dd8623128eb5b3f0f2c
-  CocoaAsyncSocket: eafaa68a7e0ec99ead0a7b35015e0bf25d2c8987
-  CocoaLibEvent: 2fab71b8bd46dd33ddb959f7928ec5909f838e3f
-  DoubleConversion: e22e0762848812a87afd67ffda3998d9ef29170c
-  Folly: ac702dfbe9294be1002f9cddaa0c2ddb99b16288
-  glog: 1de0bb937dccdc981596d3b5825ebfb765017ded
-  OpenSSL-Static: bd17e34564a8591ad76b740318683a6caa19a13e
-  PeerTalk: 77481b0a8136f226b90ccf828d6061f70139ffde
-  RSocket: 4fdb7e562db30a2d4fceddefdc601749ffc9ebe2
-  Sonar: 38a48b88369cfa48f1a36150d57e2a72ef6fd33c
-
-PODFILE CHECKSUM: d3418576aca52cddf5a9c1c7f61e1b7c606caba4
-
-COCOAPODS: 1.5.2