Add a sprite generation script to Phabricator

Summary: This probably needs some tweaks, but gets the point across. I'll lay in the actual usage in the next diff.

Test Plan: Looked at generated CSS/PNG.

Reviewers: btrahan, chad, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3278