Trying out pretty-printing of XML results

This is using Google's pretty-printer, called externally. Will pull this
in locally if it works out OK.
AssessmentRender now does explicit XML pipeline.

Serialization is now a separate step. This makes the XSLT a bit
cleaner and would also allow us to do theming later, if required.
Minor fix to SQL migration script
AssessmentRenderer now renders to a JAXP Result

This is a bit more flexible. Consequently, there's now an
option to choose the encoding if rendering to a StreamResult
wrapped on an OutputStream.
Tightened up setting of outcome variables.

Some expressions were setting OVs directly in the state,
which was not working correctly for completion_status.
This is now fixed, and I've updated the JavaDoc for
ProcessingContext to warn against this mistake in future.
Merge branch 'test_implementation'
Merge branch 'master' into rendering_refactoring
WIP: Started new author/debug rendering