Failing runs exit

As a part of Wraith-View interface, it was hard to see when a Wraith job was actually showing bad shots.  If the job failed because of a Docker issue, it would be clear, but not if the job ran and had bad diffs.  The exit code would be 0 and show success even if the shots had differences.  This PR adds support for and exit code of 1 if any pixel difference is detected.  

What the result would look like in Wraith-View based on a detected difference.

Home page
![screenshot 2014-12-07 19 21 11](https://cloud.githubusercontent.com/assets/1492067/5332216/424f221a-7e46-11e4-937d-c5f318da1f57.png)
Job page
![screenshot 2014-12-07 19 24 10](https://cloud.githubusercontent.com/assets/1492067/5332227/a9f7d24a-7e46-11e4-9645-3a5cb35449aa.png)