Increased warning levels to -Wall -Wextra, and fixed Xcode 7 beta issues

Summary:
@public

I've increased the warning levels in the OSS frameworks, which caught a bunch of minor issues. I also fixed some new errors in Xcode 7 relating to designated initializers and TLS security.

Test Plan:
* Test the sample apps and make sure they still work.
* Run tests.

React/Base/RCTJavaScriptLoader.m

@@ -21,12 +21,16 @@
 
 - (instancetype)initWithBridge:(RCTBridge *)bridge
 {
+  RCTAssert(bridge, @"birdge parameter is required");
+
   if ((self = [super init])) {
     _bridge = bridge;
   }
   return self;
 }
 
+RCT_NOT_IMPLEMENTED(-init)
+
 - (void)loadBundleAtURL:(NSURL *)scriptURL onComplete:(void (^)(NSError *, NSString *))onComplete
 {
   // Sanitize the script URL