This makes the code a bit more self-documenting

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.

This allows them to share the same describe block

This whole pull request is essentially a revert of 577e2386 and #11

Update nokogiri dependency >= 1.5

This brings OmniAuth-CAS more in line with other strategies like the
built-in Developer strategy, OmniAuth-Shibboleth, OmniAuth-SSO-Cookie,
etc.

Following http://guides.rubygems.org/patterns/

A previous update forcing only Nokogiri 1.6 breaks several
applications.  Since Nokogiri 1.5 still works just fine I
modified the dependency to be >= 1.6 which accounts for
Nokogiri 1.6 as well.

Support JASIG style extra attributes

Upgrading nokogiri dependency to ~> 1.6