add guard for asset compilation. finishes #3

This implements issue #3 

It works well here on my machine but I'd love a quick code review. The new instructions in the README explain how to use it. It should handle compilation of the coffeescript and sass assets but *also* the jekyll building and serving. If it does that for you and seems useful, let me know and I'll merge it :)

![screen shot 2013-11-01 at 7 16 08 pm](https://f.cloud.github.com/assets/1421211/1457853/a63185c0-434b-11e3-8dbf-9d399ec3262d.png)