Convert Dashboard to Rails Engine

I just started using Bandit and liked how simple it was up until I had to dump a bunch of files in my project to get the Dashboard. So I converted it into a Rails Engine that you can just mount.

There's only a few actual code changes besides namespacing:

* Converted assets into a more "pipeliney" way (using manifests)
* Cleaned up the views slightly
* Re-did the README in Markdown for syntax highlighting

I can easily remove the last change if you prefer RDoc though.