Minor tweaks found during i18n work.
Use the same marker everywhere for translator comments.
Mark XModule strings for i18n
Lorem is now fancy, and HTML tags are recognized with single-quote attributes.
Use ddt to separate test cases into tests.
Fix plural handling, put teeth in msgfmt -c test.
More validation of .po files: check message structure

The tests in test_validate.py now examine the messages to see that they
have the right structure.  The English and the translation should have
the same tags, the translation can't be blank, and it can't have any
Astral plane characters.