Made responseHandlers thread safe.

responseHandlers can be accessed from a non-main thread when the NSURLProtocol is used. It needs to be guarded by a monitor.
Stop sending messages to NSURLProtoclClient after stopLoading.

Current behavior breaks the NSURLProtocol contract because it continues to send messages to the NSURLProtocolClient after stopLoading.
Merge branch 'thread_safe_response_handlers' into master2
Merge branch 'cancel_during_responseTime' into master2

Conflicts:
	OHHTTPStubs/OHHTTPStubs.m
		Kept master2, but added !self.stopped guards around execute_after calls in non-error case.