R1:309e40b4d70b

Adds a CSS dropdown caret for buttons.

Summary: I'll build out a PHUI Class for this soonish.

Test Plan: UIExamples

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6023