Major refactor of message attachment handling; no longer stores verbatim
message copy in database.

Bumping version number to 2.1

* Walks through incoming message, write attachments to disk as they are found,
  and alters message body removing actual attachment body, and adding
  header 'X-Django-Mailbox-Interpolate-Attachment' referencing the ID of the
  stored attachment.
* When calling ``get_email_object``, will walk through stored message, and
  're-hydrate' the message by finding said headers, searching for the
  appropriate record in the MessageAttachments table, and rebuild the message
  object as closely as possible.

Minor fixes:

* Properly collect text/plain content from any part of the message; previously
  would only check the message's first level of payloads, now walks through
  all payloads, building a string of all text/plain content.
* Remove use of deprecated `assertEquals` message.
Added tag 2.1 for changeset 4d72a7d6bb96