Adding slugify utility.
Matching parent posts with cleaned email.in_reply_to ids.

Some mail clients will set the email header 'in_reply_to' as something like
`'Kevin\'s message of "Mon, 05 Aug 2013 11:33:54 PDT."\r\n<CAGVXcSad0o0ZYNDq2DnnJJ5EL1vkrk2GmQWJz3SfqpJ=iEOG=g@mail.gmail.com>'` instead of just
`<CAGVXcSad0o0ZYNDq2DnnJJ5EL1vkrk2GmQWJz3SfqpJ=iEOG=g@mail.gmail.com>`. Now
an email's `in_reply_to` is cleaned so it can be threaded properly.
Using `repr` to show unicode in __repr__ methods.
Removing typo from Extract.__repr__.
Using decimal placeholders in Post.__repr__.
Using discussion.topic as discussion.slug when slug is empty.

The slugify function can handle accented characters, but maybe it's not quite
what we're looking for.
Deleting ToC when the discussion is deleted.
Sending data['inbox'] = discussion.total_posts() in posts api.
Getting posts for parent_id where available in posts api.
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop