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.
