Clearance controllers publicize themselves as just that

* This allows performing actions on controllers conditionally, such as
[checking authorization using cancan](https://github.com/ryanb/cancan/wiki/Ensure-Authorization#wiki-conditionally-check-authorization) for all controllers except for the
ones in clearance.
* Adds a BaseController

The desired behaviour would be to accomplish something like this:

    class ApplicationController < ActionController::Base
      check_authorization unless: :clearance_controller?
    end