Add Mongoid support

**As a** developer 
**In order to** add token authentication features when working with [Mongoid][mongoid] 
**I want** _Simple Token Authentication_ to support it (see #38)

**In order to** [provide an example][example] of how to extend the gem to support different ORM/ODM/OxM 
**I want** to add support for Mongoid using the [adapter interface][adapter] (see #97)

  [adapter]: https://github.com/gonzalo-bulnes/simple_token_authentication/blob/master/spec/support/spec_for_adapter.rb
  [mongoid]: https://github.com/mongoid/mongoid