add better_errors to improve debugging experience

![687474703a2f2f692e696d6775722e636f6d2f7a594f58462e706e67](https://f.cloud.github.com/assets/688886/368972/8c8e3628-a2cd-11e2-8777-46161a52538b.png)

This is in-browser REPL that lets you execute code and inspect local variables at __each point__ in the stack trace.

'nuff said