Added a mailer preview and improved break/success emails

The number one feedback from the emails is that the subject line wasn't straightforward enough. In addition to addressing that and small layout changes, emails now include timing stats that compare sharded time to linearized time.

Also added fancy Rails email preview class.
Ideally we would create a real git repository with fake commits during `db:seed`. Then we wouldn't need to stub GitBlame.

I think that if you visit `rails/mailers` and then visit normal pages after, the faked `GitBlame` class will still be loaded. I don't think this will be a problem for now because GitBlame is only used inside Resque jobs.