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 passed on this,  but add #623 to address the `failed` counter option.  Also, I've added #624 in case we'd like to use the `Messages` class for all email-related communication.