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

docs/getting-started.md

@@ -97,11 +97,12 @@ swift_version = "4.1"
 flipperkit_version = '0.8.1'
 
 target 'MyApp' do
-
+  platform :ios, '9.0'
   pod 'FlipperKit', '~>'+flipperkit_version
   # Layout and network plugins are not yet supported for swift projects
   pod 'FlipperKit/FlipperKitLayoutComponentKitSupport', '~>' + flipperkit_version
   pod 'FlipperKit/SKIOSNetworkPlugin', '~>' + flipperkit_version
+  pod 'FlipperKit/FlipperKitUserDefaultsPlugin', '~>' + flipperkit_version
 
   post_install do |installer|
 
@@ -120,6 +121,7 @@ and install the dependencies by running `pod install`. When you open the Xcode w
 
 ```objective-c
 #import <FlipperKit/FlipperClient.h>
+#import <FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h>
 
 @implementation AppDelegate
 
@@ -130,6 +132,8 @@ and install the dependencies by running `pod install`. When you open the Xcode w
   SKDescriptorMapper *layoutDescriptorMapper = [[SKDescriptorMapper alloc] initWithDefaults];
   [client addPlugin:[[FlipperKitLayoutPlugin alloc] initWithRootNode: application
                                                   withDescriptorMapper: layoutDescriptorMapper]];
+  [client addPlugin:[[FKUserDefaultsPlugin alloc] initWithSuiteName:nil]];
+
   [client start];
 #endif
   ...