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.

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -43,7 +43,9 @@ RCT_EXPORT_MODULE()
 
 - (instancetype)initWithURL:(NSURL *)URL
 {
-  if (self = [super init]) {
+  RCTAssertParam(URL);
+
+  if ((self = [super init])) {
     _url = URL;
   }
   return self;