Replacing 'to_address' and 'from_address' on email.

Using 'recipients' and 'sender' respectively.
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Adding method post.ancestors()

Returns a list of the post's ancestors from eldest to youngest.
Fixing post.last_updated() when the post has no descendants.

Returning post.content.creation_date in that case.
Fixing discussion.posts()

First, the function caused an error in Postgres, then I realized it wasn't
sorting the results the way I'd expected. It works now using SQL's COALESCE in
`latest_update`. If a post doesn't have any descendants, it's latest_update is
the date that its content was created.
Renaming mailbox.mailbox to mailbox.folder.
Changing Unicode to String type on mailbox.host.

Otherwise the socket won't be able to reach the host.