Authenticator rewrite

- Next version is now [2.0](https://groups.google.com/forum/#!topic/play-silhouette/Jv2sYa7NiJg)
- Add JWTAuthenticator
- Rename HeaderAuthenticator to BearerTokenAuthenticator
- Allow to renew an authenticator
- To create an authenticator only the login info is needed instead of the complete identity
- Sliding window expiration can now be disabled
- Authenticators now using an own backing store implementation instead of the cache
- Update documentation