Issue #15 more options for CAS...

More options for values handed back by CAS.  This is needed because
different auth strategies for CAS sometimes hand back different
keys for these items.  They're not configurable on an application
basis.  Only globally.  This means omniauth-cas needs to be more
flexible and setting defaults while you're at it.
Updating documentation for new CAS options
Minor grammar change to README.
Fix option code for failing test