Feature/auth

Auth appears to be working now. Tried basic auth with:

http://httpbin.org/basic-auth/user/passwd

And digest auth with:

http://httpbin.org/digest-auth/:qop/:user/:passwd

Both worked properly. I just have some docs to finish cleaning up.
Do we have to make it hard set to permanent? Seems like it should be configurable in the auth object. I would recommend creating a property of `NSURLCredentialPersistence ` and using a smart default value. The it can be overridden if needed.