Use AFNetworking and add support for retrieving fitness activities

ASIHTTPRequest is obsolete for a while now and AFNetworking is the way to go in terms of networking on iOS and Mac OS.

I replaced ASI and SBJSON with using AFNetworking. I also use AFNetworking and OAuth2Client as submodules to make it easier going forward to pull changes.

Lastly, I added support for retrieving the fitness activity feed and the fitness activity summary for an activity.

Adding support for other API end points is super simple now but I don't need any more data at this time.