Added email archiving option to IMAP transport

Similar to @eamonn_faherty in [Issue #9 Deleting Emails](https://github.com/coddingtonbear/django-mailbox/issues/9) I am glad to have found this library (thanks for writing it!), but I wanted a way of keeping an archive of all emails on the mail server as well. One simple solution to this problem is to move the emails that were read into an archive folder. The IMAP protocol allows to do that in a simple way.

So in this pull request I added support for that by adding a query parameter to the uri, which allows the user to specify an archive folder name for the IMAP transport. I added tests and updated the documentation accordingly.

This is not a global solution for the problem, as it only works for IMAP, but maybe its useful anyway.