Various enhancements for Gaku::User:

* Remove defunct Gaku::User#has_role?
* Refactor and fix logic for Gaku::User#role?
* Add specs for all scopes and methods #to_s, #role?
* Document Gaku::User#find_first_by_auth_conditions
Merge branch 'master' of github.com:Genshin/gaku into add-tests-for-users
Merge pull request #319 from Genshin/add-tests-for-users

Various enhancements for Gaku::User: