improve coffee compilation performance

- uses grunt-newer to selectively recompile only those coffeescript
files that have changed since the last compile
- timestamps to determine whether to recompile are cached in
`generated/timestamps` so they will be removed via `lineman clean`
- this enables some impressive performance gains, as coffeescript recompilation only acts on those files that are changed, yet all previous files are merged into the bundle.

![screen shot 2013-10-16 at 10 39 22 pm](https://f.cloud.github.com/assets/69559/1348811/8de3b7a4-36d5-11e3-8cb4-9880c3f5aab1.png)

/cc @searls @jasonkarns (could you guys test this out for me? we've been using a similar config in shopify, and it's improved compilation times dramatically).