Remove non-ARC support and enable ARC for the iOS Target

We weren't using ARC for `OHHTTPStubs-iOS`, and that meant we were seeing some crashes in iOS test targets.

I've removed the non-arc support code, and enabled ARC for all targets. Who doesn't use ARC these days anyway? :trollface: 

Also improved the thread safety of `_requestHandlers` a little bit, and tidied up some style.