Add last_modified to Contact API

For integrating with external contact data, a client requested addition of a last_modified field in CiviCRM Contact API.

Initial work (seems functional) for 4.3 here:

https://gist.githubusercontent.com/xurizaemon/c792897ebc9e5b73dc85/raw/628daf34c4fa40c8b1a28b5669fc694baf2ce657/civicrm-last_modified.patch

Adds return 'last_modified' to Contact API, retrieves MAX(modified_date) for the matching entity from table civicrm_log & adds to results.