Relative date helper, for 'today' and 'yesterday'

Summary:
Format a date as 'today', 'yesterday', or 'Mar 27 2012'. Optionally,
the final example can be rendered 'on Mar 27 2012' for things like:

$excuse =
  'I fell out of a window '.
  phabricator_on_rel_date($time, $me);

Test Plan: Tested in my sandbox!!!!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2035