Play Authenticate for Play! 2.3

I have migrated play-authenticate to Play! 2.3.

Changes:

1. Replaced Build.scala with build.sbt.
2. Replaced play.mvc.SimpleResult with play.mvc.Result.
3. Migrated to the new package structure of libs.ws, libs.oauth, and libs.openid.
4. Use public fields instead of methods to access fields on OAuth.RequestToken instances.
5. Use different exception handling mechanism when calling OAuth.retrieveAccessToken(). **This change should be reviewed.**
6. OAuth1AuthProvider#signedOauthGet(): Changed WS call. **This change should be reviewed.**
7. OpenIdAuthProvider#authenticate(): Changed Promise.get() call. **This change should be reviewed.**