Okhttp Oauth2 Client Versions Save

Remove backup from manifest

Switch from Gson to Moshi

Fixes issue #1 and #3 -- thanks to @eknoes!

• Proguard rules

• Added callback
• Capture exception if it exists in OAuthError

This is a major (and likely last) refactor of the initial 1.0.0 release. It includes:

• Removal of OAuthException
• Better handling of errors and success/failure details