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
Join multiline comments by ' '. Each tag starts new comment.

With multiple single line comments, if they both have tags
we want to remove them correctly if we have strip_comment_tags=True.

I've joined multiline comments by ' ' instead of '\n'.  This means
we can't add a \n in a comment if we wanted to, but the general
case when adding a multi-line comment in a file is that we do want
to treat it as a single comment. This will make it easier to merge
po_entries from two different files.

Test Plan:
tools/runtests.py tools/runtests.py third_party/babel/messages/tests/extract.py exercises.babel_test.ExtractStringsTest kake.compile_all_pot_test

Auditors: csilvers