Keep comments in capa XML from causing failures

Comments (and processing instructions!) are handled oddly in lxml.
This change will keep them from causing failures.  They will be omitted
from the HTML generated, which is fine, since they aren't needed there.
A test that our XML-comments fix works.
Merge pull request #805 from edx/ned/fix-allow-comments-in-capa-xml

Keep comments in capa XML from causing failures