CRM-12556 - api/v3 - Identify contacts by user name

(Resubmit https://github.com/civicrm/civicrm-core/pull/709 based on upstream/master)

Per IRC discussion, this addresses http://issues.civicrm.org/jira/browse/CRM-12556 by introducing an API feature that translates an expression "@user:admin" to a contact ID. This seemed like a useful generalization of the problem from CRM-12556.