Remove cas

There are two commits in this.  
The first part is a small modification to the plugin initializer to support using a forked gem.  It allows setting the name for the 'require name' call if that name is different than the name of the gem.  In my case that is erik-omniauth-cas vs omniauth-cas.  I needed to do this as I am still waiting on the omniauth-cas developer to release a new version.  I am tired of waiting so I will maintain a fork for now. 

The second part is more invasive and removes all the CAS code and converts that settings over to be of use by the plugin.  

Before adding this we need to announce that CAS is moving out of core.  It maybe that I am the only one using this so I am not sure how loud to be.