Added support for additional SAML token fields to the info hash in the callback phase.

This pull request is for Issue #28 on the PracticallyGreen/omniauth-saml repository. It adds an expanded number of info hash fields during the callback phase. The fields are pulled from the ruby-saml Response class. The updated fields include:

* issuer 
* session_index
* session_not_on_or_after
* response.session_expires_at
* not_before
* not_on_or_after

An additional test case was added to validate the new fields. 
Good catch Joe. I'll resubmit.