Add a description/toggle to PHUIObjectItemView

Summary: Gives the ability to hide a big long block of text in an ObjectListItem without cluttering the UI.

Test Plan:
Added a test case to UIExamples. Click on icon, see content. Click again, content go away.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18006