add related object, mass mail utils

I added a generic foreign key field on Message in order to allow relating sent or received messages to any other object. For instance, I use this to process email replies to comment notifications. (I only added the south migration so far.)

I also added send_mass_mail functions (for text-only and multi-alternative with html) that store outgoing messages in the Message table and accept a related object to be stored in the aforementioned new column.