Add a very crude spriting pipeline to libphutil

Summary:
There are a bunch of sprite tools out there, but I want a very high degree of control and repeatability in spriting: specifically, when Chad makes an icon look really pretty, I just want to Select All -> Paste Layer Style -> Save For Web... and then run a script and have everything work. This means reapeatable, scripted generation from multiple large source inputs, which none of the web tools can do.

Since I knew the amount of code involved in getting exactly what I wanted wasn't large, I just wrote it after poking around a bit on Google without finding any hidden gold.

Test Plan: Generated sprites. See next diff.

Reviewers: btrahan, chad, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3277