Add Base Grey set of colors

Summary: This adds a set of standard grey colors for use in shading objects and importance. I'll follow up and start implementing in another diff.

Test Plan: Color UI Examples, Adobe Kuler

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6853