cache builds locally.

[bukzor]

@springmeyer and I discussed this at some length on freenode #mapbox last Friday.
Here are my notes: bukzor@c6481ae

The general idea is that if you build something with node-pre-cache twice, the second build should be cached and very quick, even if the upstream manager has not published a build for your platform.

[bukzor]

This is my attempt to add a put-to-cache just after build.
I'm not familiar with nodejs best practices (or even js really), so please tell me what I did wrong :)

bukzor@9ec7f64

[springmeyer]

Looking quite good so far. Can you queue up as a pull request?