CRM-11823 - Enable CiviCRM to generate HTML HEAD elements on non-Civi pages

More details at:

http://issues.civicrm.org/jira/browse/CRM-11823
http://wiki.civicrm.org/confluence/display/CRMDOC43/HTML+Header+Region

Note: This PR is tightly-coupled to PR's for each of the integration modules.