Rework single sign out support to work in Rails 3.1/3.2

This pull request is to address problems with the single sign out approach to work in Rails 3.1/3.2.  For more information see this issue:
https://github.com/nbudin/devise_cas_authenticatable/issues/47#issuecomment-5244557

This pull request switches the approach from trying to monkey patch the session store and instead requires a filter to be added to ApplicationController like so:
include DeviseCasAuthenticatable::SingleSignOut::StoreSessionIdFilter

I'm mostly sending this pull request to start a discussion on the right approach. A couple of questions: 
1) should we automatically include this filter in ApplicationController if it's defined?
2) should we be using Rack middleware instead?