Added support for refresh tokens. Android support not added or checked.

I believe I have refresh token support working. I do not have an Android subscription to test/work with. This is ONLY the iOS support. I have tested it in multiple situations and it seems to be working. NOTE: in order for it to work on a device, I had to set my "Custom Login Screen" in the Auth0 Dashboard to use the 5.2.7 version of screen. Otherwise it doesn't populate the loading screen values correctly.