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'm questioning the logic here.  In `#update_file`, you cycle through all the files and only if you have permission to edit every file, will it proceed.  So you should never have `denied` and `saved` at the same time.  Also, the rescue block should skip any files that weren't updated?  The problem there is there's just an exception raised... does that kill the whole thing or just that one file?  and, is it reported to the user at all?  Do we need an additional `failed` counter to convey failures, as files that you do have access to but couldn't updated?