Give Attachments the right to children
Set on_delete on ForeignKeys that we don't want CASCADE_DELETE on
Attachments will now store binary data correctly.

Fix #48
Fix regression in previous commit for issue #48
Changes for Django 1.6

Fixes #49
Add rendered property to BlogPost to render to HTML
Reorganise models and add unique flags back in
New email models

Fixes #38
Headers can carry a lot of data for no good reason, but we should support that anyway
Spelling error and remove pre-delete signal
Protect certain models from cascading deletes
More fiddling :P
Add managers to Body and Header to make new models nicer to interact with
Have part and header numbers rather than using relations
Change get_or_create to create (because that's actually what it does :P)
Changed my mind about ordinal being null again
PartLists need to be related to emails
Use BitField

Fixes #56
Remove admin and Inbox helpers, add Inbox manager
Move available inboxes code to UserProfile model