Treat multi-line comments, and multiple single line comments as one.

Summary:
Since we end up sorting comments we saw behavior such as in:
http://phabricator.khanacademy.org/D10185 where the comment lines got
out of order.  But besides our issue with sorting, it kind of makes
sense to treat these comments as a single comment because they do
all go together.

Test Plan: tools/runtests.py tools/runtests.py babel.messages.tests.extract.ExtractJavaScriptTestCase

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D10197