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/Views/RCTPickerManager.m

@@ -27,7 +27,7 @@ RCT_EXPORT_VIEW_PROPERTY(selectedIndex, NSInteger)
 
 - (NSDictionary *)constantsToExport
 {
-  RCTPicker *view = [[RCTPicker alloc] init];
+  UIPickerView *view = [[UIPickerView alloc] init];
   return @{
     @"ComponentHeight": @(view.intrinsicContentSize.height),
     @"ComponentWidth": @(view.intrinsicContentSize.width)