Compatible with OAuthLib 0.6.2 and 0.6.3.

https://github.com/lepture/flask-oauthlib/issues/100
Add error message for validate_bear_token on oauth2.

https://github.com/lepture/flask-oauthlib/issues/110
Add `invalid_response` decorator for custom error response on
`require_oauth`.

https://github.com/lepture/flask-oauthlib/issues/104
https://github.com/lepture/flask-oauthlib/issues/110