Check if cas:attributes exists in xml_from_dict exists before trying …

Fixes

Fix the bug because of the new version of [Flask](https://pypi.org/project/Flask/) not support `flask.ext.cas` semantics

Check if cas:attributes exists in xml_from_dict exists before trying to get the attributes or memberOf values in the event that the CAS server/services doesn't present attributes.

Issue #31. Solved bug when no attributes.

Using a default value when CAS give no attributes.

Fix link to CAS documentation

Jasig has been renamed to Apereo, and the official CAS site has been moved to GitHub.

Add a Gitter chat badge to README.md

Utilize serviceValidate endpoint available on CAS 3.0

Logout fix

'url' parameter changed to 'service' according to
http://jasig.github.io/cas/development/installation/Logout-Single-Signout.html

Added CAS_AFTER_LOGOUT config to specify URL for redirect after logout

fixes for 2.6

fix for python2.6