create sprite sheet bundling tool

[incanus]

For the time being, we will need an easy-to-use tool for devs to pre-bundle marker images into a style's sprite sheet, until we allow runtime imagery and even animations for markers (see #877).

We have ./bin/build-sprite.js from https://github.com/mapbox/mapbox-gl-js#sprite-generation but we should think about distributing a tool in this repo and/or making an easy download of this standalone.

[jfirebaugh]

We should move build-sprite.js to mapbox-gl-style-spec, our soon-to-be-monolithic-again repo for style spec and associated utilities.

[incanus]

Yeah, was almost gonna suggest that. Makes sense to have it there.

[ljbade]

We will also want some documentation/simple tutorial on how to get started work custom markers