Add a Phurl Typeahead

Summary: Adds a basic typeahead for Phurl Objects.

Test Plan: http://local.phacility.com/typeahead/browse/PhabricatorPhurlURLDatasource/

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17339