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/UserDefaultsViewController.m

@@ -0,0 +1,30 @@
+//
+//  UserDefaultsViewController.m
+//  Sample
+//
+//  Created by Marc Terns on 10/7/18.
+//  Copyright © 2018 Facebook. All rights reserved.
+//
+
+#import "UserDefaultsViewController.h"
+
+@interface UserDefaultsViewController ()
+@property (weak, nonatomic) IBOutlet UITextField *valueTextField;
+@property (weak, nonatomic) IBOutlet UITextField *keyTextField;
+@property (nonatomic, strong) NSUserDefaults *userDefaults;
+@end
+
+@implementation UserDefaultsViewController
+
+- (instancetype)initWithCoder:(NSCoder *)aDecoder {
+    if (self = [super initWithCoder:aDecoder]) {
+        _userDefaults = [[NSUserDefaults alloc] initWithSuiteName:nil];
+    }
+    return self;
+}
+
+- (IBAction)tappedSave:(id)sender {
+    [self.userDefaults setObject:self.valueTextField.text forKey:self.keyTextField.text];
+}
+
+@end