better_errors integration

Add "better_errors" gem for easier debugging: https://github.com/charliesome/better_errors

![687474703a2f2f692e696d6775722e636f6d2f7a594f58462e706e67](https://f.cloud.github.com/assets/162970/492903/6c587e00-baff-11e2-92a0-23ab57a557d5.png)