Add options hash to message model

This should allow passing of other options forward to the Message model
where it can be used.

I would like to track more then one thing per message. For example my store has users and cat clocks. I would like to know the open rate for a user (via the user option currently) and the open rate for my Hello Kitty Kit-Cat clocks. I am thinking I could pass an option of tracking: [clock1, clock2] and I would make my own Message class that would intercept the tracking option. After creation of a message it would insert some records in to a join table. 

If there is a different/better way to tracking many things per message please let me know.
Thanks!