Summary:
@public
Add `RCTAssertThread` to `RCTAssert.h` for convenience when checking the current/queue,
it accepts either a `NSString *`, `NSThread *` or `dispatch_queue_t` as the object to be checked
Also add a check to `-[RCTUIManager addUIBlock:]` - There was a discussion on github (https://github.com/facebook/react-native/issues/1365)
due to the weird behavior caused by calling it from a different thread/queue (it might be added after `batchDidComplete` has been called
and will just be dispatched on the next call from JS to objc)
Test Plan:
Change `-[RCTAnimationExperimentalManager methodQueue]` to return `dispatch_get_main_queue()` and run the 2048 example,
it should dispatch with a helpful message (screenshot on the comments)
