Build XML responses with the Element module

This eliminates the need for XML templates to handle service
responses.
Add tests for XML response generation classes
Remove XML parsing from views tests

The output XML is parsed and tested within the response tests. For
views, just check that the response contains the appropriate text.
Update documentation for XML output changes
Expand comments for XML responses
Output CAS 2.0 XML responses as UTF-8
Improve comments and minor reformatting
Add default parameters to response init