Allow users to execute responses arbitrarily.

Instead of relying on a timeout, users can now execute responses arbitrarily. An OHHTTPStubsResponse has a responder block, which if populated is called with a dispatch_block_t, which can be called from anywhere to cause processing to continue. This makes ordered operations much easier to guarantee, and will make tests execute faster.
Stop sending messages to NSURLProtoclClient after stopLoading.

Current behavior breaks the NSURLProtocol contract because it continues to send messages to the NSURLProtocolClient after stopLoading.