Add basis attachments to todos

This PR implements an attachment model, referenced from a todo. (1-n todo - attachments)

Currently only the MessageGateway attaches the received email to the newly created todo. (This scratches my personal itch). We need a ui for uploading attachments to todos. This is the start

Open for discussion:
* inclusion for 2.3 or later
* paperclip or not
* how to handle user data outside of the database (attachments on file server). Instructions for backup. Data integrity
* what todo with the archived todos that have (big) attachments. Delete on completion or leave it to the user?