Using Sufia::Messages for notifications

This is to address a problem where a long list of files is displayed for a batch upload:
![bad](https://cloud.githubusercontent.com/assets/312085/3802348/30b45e34-1c0e-11e4-9a44-a51656187d45.png)
Using the popover feature in Bootstrap, it'll look something like:
![good](https://cloud.githubusercontent.com/assets/312085/3802357/54e188cc-1c0e-11e4-8947-d1ee0d81d9d1.png)
and the files are displayed in the popover when the user clicks on the link, and they can click and individual file from there.

To do this, I refactored `BatchUpdateJob` to include the new class `Sufia::Messages` which mixes in some html-ish-ness.  I'm not sure about this solution, as it's mixing in html display stuff into a class that's under `lib`. Another option would be to use Rails' `AbstractController` and use an actual view to render the html that's going in the email message.  Thoughts?

Also, note the `sanitize_text = false` option in `#send_message`.  This is needed to render the complex bits of html, otherwise, it's just removed.

Comments?
@jcoyne @mjgiarlo @cam156 


I wonder whether we should opportunistically tweak lines like this to use the newer rspec syntax so we chip away at technical debt rather than leave it out there, or add to it, under the assumption that we're going to take care of this stuff in one fell swoop later.  I'm on the fence about this -- throwing it out there for comment.