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 
Hi @nicolo:

I would say `X-SuperAdmin-PhoneNumber`.
Independently from the expectation we agree on, I believe that these names generators are becoming very complex and should be tested on their own. I'll copy this branch to add RSpec so we can write some unit tests; they will make the development easier.

@ProGM I'll cherry-pick your commits, I suppose that's ok, but please let me know if it's not!