Threshold for failure

Set a threshold for failure in a config file.  This will be useful for people who want to ignore small changes such as dynamic tweet numbers or external changes.  The gallery has been updated to add a cross when a fail is detected so that locating shots is easier.

![post-13513-star-wars-warp-speed-gif-doof](https://cloud.githubusercontent.com/assets/1492067/5340993/3ef945ba-7eea-11e4-924b-1d8dbcb41453.gif)