Fix error message for ODK-only question types

Use the question type name rather than "Image capture".

http://manage.dimagi.com/default.asp?139419
Merge pull request #259 from dimagi/139419-odk-error

Fix error message for ODK-only question types
Show unified diff on assertXmlEqual fail

Much easier to read than full dumps of actual and expected XML.
Allow hidden values in groups

Tests are failing! No drag/drop support yet.
Fix “adds all question types and attributes” test

Changes to tests/static/all_question_types.xml are whitespace only (to improve diff output).
Fix remaining failing tests
Clean up tree 'into'/'inside' inconsistency
Use mug.options.isDataOnly where possible

Instead of comparing mug.__className to "DataBindOnly"
Implement drag+drop hidden values into group
Handle 'into' and 'inside' in move check