JWT and Bearer tokens support on both dev and production.

endpoints.CurrentUser() method now tries to validate id_token (JWT)
and falls back to bearer token.

- fixes #2: Get user information from the id_token or
            oauth token in the request
- fixes #6: Use tokeninfo API for Bearer token validation
            in dev_appserver

Reviewed in https://github.com/crhym3/go-endpoints/pull/15