Integrating NSUserDefaults plugin to iOS (#291)

Summary:
I have a few details left, but its almost done. This PR addresses #145

- The NSUserDefaults plugin uses the SharedPreferences Desktop Part since we can reuse all of it.
- The NSUserDefaults plugin uses swizzling in order to be notified of what specific event changed at runtime.
- Added Test harness in both Sample Swift and Sample apps for iOS in order to test the plugin.
- Updated the documentation in `docs/shared-preferences-plugin.md` and` README.md`

I am open to suggestions since the desktop sharedPreferences version doesn't support deletion of preferences. Most likely I would have to modify the UI, and for that matter, I might as well build a user defaults desktop version

I wanted to add xiphirx in this MR since he developed the shared preferences plugin for Android and Desktop. I don't see a way to remove preferences from the flipper desktop app so I was wondering if you would be OK with me adding that.
Pull Request resolved: https://github.com/facebook/flipper/pull/291

Reviewed By: passy

Differential Revision: D10334685

Pulled By: priteshrnandgaonkar

fbshipit-source-id: d798c01a46df7ddecf713924799f046b560ea922

iOS/FlipperKit.podspec

@@ -63,6 +63,7 @@ Pod::Spec.new do |spec|
                              'iOS/Plugins/FlipperKitNetworkPlugin/FlipperKitNetworkPlugin/SKRequestInfo.h',
                              'iOS/Plugins/FlipperKitNetworkPlugin/FlipperKitNetworkPlugin/SKResponseInfo.h',
                              'iOS/Plugins/FlipperKitNetworkPlugin/FlipperKitNetworkPlugin/FlipperKitNetworkPlugin.h',
+                             'iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h',
                              'iOS/FBDefines/FBMacros.h',
                              'iOS/FlipperKit/**/{FlipperStateUpdateListener,FlipperClient,FlipperPlugin,FlipperConnection,FlipperResponder,SKMacros}.h'
     header_search_paths = "\"$(PODS_ROOT)/FlipperKit/iOS/FlipperKit\" \"$(PODS_ROOT)\"/Headers/Private/FlipperKit/** \"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/PeerTalkSonar\""
@@ -131,4 +132,13 @@ Pod::Spec.new do |spec|
     ss.source_files         = "iOS/Plugins/FlipperKitNetworkPlugin/SKIOSNetworkPlugin/**/*.{h,cpp,m,mm}"
     ss.pod_target_xcconfig = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)\"/Headers/Private/FlipperKit/**" }
   end
+
+  spec.subspec "FlipperKitUserDefaultsPlugin" do |ss|
+    ss.header_dir = "FlipperKitUserDefaultsPlugin"
+    ss.dependency 'FlipperKit/Core'
+    ss.compiler_flags       = folly_compiler_flags
+    ss.public_header_files = 'iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h'
+    ss.source_files         = "iOS/Plugins/FlipperKitUserDefaultsPlugin/**/*.{h,m}"
+    ss.pod_target_xcconfig = { "HEADER_SEARCH_PATHS" => "\"$(PODS_ROOT)\"/Headers/Private/FlipperKit/**" }
+  end
 end