[2.0] Change AFHTTPClient API to Return Tasks Without Starting Them

It makes sense to automatically start tasks for `GET` / `POST` / et al convenience methods, but for the long-form task creation methods, there are likely to be situations where automatically calling `resume` is not desired behavior. This also brings the API more in-line with `-HTTPRequestOperationWithRequest:success:failure:`, in how the returned object is not automatically started or added to the operation queue.

I'm sending this pull request mostly as a way to communicate this significant API change more visibly, as well as to invite any additional thoughts on the matter.