Add capability to add attachments to mail campaigns

This adds the ``add_attachment`` method to ``MailCampaign``. It's pretty straightforward; I tested it with Rails 4 and everything seems to work smoothly. The Mail gem hasn't changed in a good long while so I don't foresee any major issues with it.

One other thing I did as part of this commit was remove all the calls to ``drop_index`` in the migration template; there is [no need](http://stackoverflow.com/questions/887590/does-dropping-a-table-in-mysql-also-drop-the-indexes no need) to do so explicitly.