Accept custom method name for current_user

Hi,

Thank you very much for this great gem.

This patch enables the user to choose the name associated with the ````current_user```` method in order to suit their own naming convention.
This can be useful when the user wants to impersonate one kind of user (let's say ````Manager````) but already has the ````current_user```` method for another type of user (````User````) and would like to replace it with the method ````current_manager````.

Kind regards,
Daniel