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/RCTJavaScriptExecutor.h

@@ -43,7 +43,7 @@ typedef void (^RCTJavaScriptCallback)(id json, NSError *error);
  * Runs an application script, and notifies of the script load being complete via `onComplete`.
  */
 - (void)executeApplicationScript:(NSString *)script
-                       sourceURL:(NSURL *)url
+                       sourceURL:(NSURL *)sourceURL
                       onComplete:(RCTJavaScriptCompleteBlock)onComplete;
 
 - (void)injectJSONText:(NSString *)script