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.
Awesome work, thanks for the contribution, @fsboehme!

As much as I love the idea of this, I'm a little hesitant to require a migration that'll add new columns to everybody's mail records -- the library has been around for quite a long time, and it's entirely possible that people have database tables with an inordinate number of records at this point; could I convince you into moving this into its own model, and instead using a ManyToManyField to relate those related objects with the Message record?