Removes collection transforms.
Adds a custom exception for failing on missing source or document.
Fix success test and fail on incomplete test.
Fixes the test that runs against an invalid xml document.
Separates the reference rewrites into an isolated test case.
Moves test into the transforms test case.
Coding style change
Mark test as an expected failure.
Merge pull request #154 from Connexions/transform-again

Transform cleanup
Add overwrite_html flag to to_html.produce_html_for_modules

Enable this flag means that the index.html will be removed before a new
one is generated.
Stop raising a plpy error when produce_html_for_module returns a message

produce_html_for_module now raises an exception if there's an error.