JSONObjectBlock doesn't allow for NSArrays

https://github.com/icanzilb/JSONModel/blob/master/JSONModel/JSONModelNetworking/JSONHTTPClient.h#L42

JSONHTTPClient seems to assume that you'll always be receiving a dictionary object. Surely it would be better to use `id` as the type?

I'd be entirely happy to make a pull request if you'd be open to this change.