Support JASIG style extra attributes

Some CAS server (like the [JASIG's CAS Server](http://www.jasig.org/cas), [CASino](http://casino.rbcas.com), …) format the extra attributes a bit different. Example:

``` xml
<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
  <cas:authenticationSuccess>
    <cas:user>psegel</cas:user>
    <cas:attributes>
      <cas:employeeid>54</cas:employeeid>
      <cas:first_name>P. Segel</cas:first_name>
      <!-- ... -->
    </cas:attributes>
  </cas:authenticationSuccess>
</cas:serviceResponse>
```

Links:
 - https://issues.jasig.org/browse/CAS-738
 - https://pypi.python.org/pypi/django-cas-provider/0.3.1 (`CUSTOM USER ATTRIBUTES FORMAT`)
 - https://github.com/biola/rack-cas/blob/master/lib/rack-cas/service_validation_response.rb#L36