CRM-15578 - crmMailingAB2 - Fix recipients field. DRY.
Cleanup
CRM-15578 - MailingAB API - Propagate deletion to any included mailings.
CRM-15578 - MailingAB API - Test recipients_update more thoroughly
WIP - Move submit() logic from client-side to APIv3

This makes it easier to re-use (e.g. submitting the constituents of an A/B
test) and easier to secure (e.g.  when using CiviMail advanced workflow).