Support phpCAS-style attributes to populate User model

Jasig CAS [supports a modification](https://wiki.jasig.org/display/CASUM/Attributes#Attributes-AccessingattributesusingphpCAS) which allows user attributes to be passed back when a ticket is validated when using CAS 2.0 protocol.

This patch is based on pull request #4.

With the setting `CAS_ATTRIBUTES`, django-cas2 can then set attributes on the User model, such as the user's name and email address, from information passed in ticket attributes, when the user logs in.  The use and name of attributes is undefined, so attribute selection is left to the administrator.

This is a common extension to the CAS 2.0 protocol.

In the absence of these attributes, django-cas2 will not set any of the additional attributes.