Add ImageHref attribute for PHUIObjectItemListView

Summary: In some cases we may want a different URI for the image on an item than the header/title of the item (like user / title). This prioritizes ImageHref over Href.

Test Plan: uiexamples

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18016