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/Sample/MainViewController.m

@@ -10,6 +10,7 @@
 
 #import "NetworkViewController.h"
 #import "RootViewController.h"
+#import "UserDefaultsViewController.h"
 
 @interface MainViewController ()
 
@@ -35,4 +36,11 @@
   [self.navigationController pushViewController:networkViewController animated:true];
 }
 
+- (IBAction)tappedUserDefaults:(id)sender {
+    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryBoard" bundle:nil];
+    UserDefaultsViewController *userDefaultsViewController = [storyboard instantiateViewControllerWithIdentifier:@"UserDefaultsViewController"];
+    
+    [self.navigationController pushViewController:userDefaultsViewController animated:true];
+}
+
 @end