add hook_civicrm_alterMailParams; add support for custom email headers

There are two parts to this patch.

First is adding a hook for an event after an email has been sent. Documentation for a hook_civicrm_postMailParams event has been created at http://wiki.civicrm.org/confluence/display/CRMDOC42/Hook+Reference#HookReference-hookcivicrmpostEmailSend.

Second is to add support for custom headers to emails. While code is able to add the custom header to the email in hook_civicrm_alterMailParams, core code to handle the new parameter was necessary in a couple of places. Documentation that there is now an option to use a 'headers' value in $params for these two mail hooks has been added to the page cited above.

If possibly, we would like this added to a 4.2 point release.