Adding coverage to diff view

Summary:
https://www.dropbox.com/s/3lt4wq7vfuuwurg/Screenshot%202014-05-06%2013.12.21.png

I only added the coverage to lines that were added as part of the diff.  We can change this later.

Test Plan: Tested on changes to make sure it was being rendered.

Reviewers: cramer

Reviewed By: cramer

Differential Revision: https://tails.corp.dropbox.com/D47597