Custom authentication parameter

Implementing the authentication parameter to be configurable. (Instead of hardcoded `email`)

Example:

    SimpleTokenAuthentication.configure do |config|
      config.auth_parameter_name = { user: 'your_parameter_name' }
    end

#68 
@gonzalo-bulnes I think that the code is working, I'm using it in my project actually...

The only thing is that I also added support for [Rails-API](https://github.com/rails-api/rails-api) (and i forgot to say it). You can decide if you like to take it or move it in another issue.