Add fallback_to_devise option

**As** a developer 
**In order to** build [safe API authentication by token][api]
**And** to keep being able to use [token authentication in non-API scenarii][non-api] 
**I want** the fallback_to_devise option to be available at a controller level 

  [api]: https://github.com/gonzalo-bulnes/simple_token_authentication/issues/49#issue-30693952
  [non-api]: https://github.com/gonzalo-bulnes/simple_token_authentication/issues/49#issuecomment-43410225

See #61
Fixes #49