Add JSON support

As alluded to on Twitter, this allows users to use JSON in request and response bodies. E.g.:

```objc
stubRequest(@"POST", @"http://server.dev/user")
    .withJSON(@{ @"name": @"robb", @"pw": @"foobarbaz" })
    .andReturn(201);
 
BOOL success = [client signUpWithUserName:@"robb" password:@"foobarbaz"];
```

I had some trouble getting [`master`](https://github.com/luisobo/Nocilla/commit/16c7bd51eabb92f88323ebaf5f18ad07cfbcb605) to build on my machine and in Xcode 6. I updated some of the dependencies to please the compiler but had to mark two tests as pending for the time being.