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.

Examples/UIExplorer/UIExplorerUnitTests/RCTConvert_NSURLTests.m

@@ -13,7 +13,7 @@
 #define TEST_URL(name, _input, _expectedURL) \
 - (void)test_##name { \
   NSURL *result = [RCTConvert NSURL:_input]; \
-  NSURL *expected = [NSURL URLWithString:_expectedURL]; \
+  NSURL *expected = (_expectedURL) ? [NSURL URLWithString:_expectedURL ?: @""] : nil; \
   XCTAssertEqualObjects(result.absoluteURL, expected); \
 } \