Use sinatra-assetpack to manage js/css assets

[cespare]

This has several advantages:

• We can get rid of a whole bunch of custom rendering/caching code
• Dev/prod differences will be handled for us
• This will enable us to easily switch to using the less gem, which means that we can remove all the node dependencies in Barkeep and simplify installation

[cespare]

I spent some time on this about 1 or 2 weeks ago, but I couldn't get it working for a variety of reasons. Maybe I just need to spend more time with it, but I didn't have a very easy time getting it to render our stuff.

Also, I was wrong about the less gem...as noted in the Gemfile, it doesn't really seem to work with 1.9. So sinatra-assetpack (which just uses Tilt to render most things) may not help us much with LESS.

[cespare]

Closing as we're not going to use this.