MDL-10107 - First check-in of Messaging 2.0 code from Luis Rodrigues, GSOC student.

MDL-10107 - First check-in of Messaging 2.0 code from Luis Rodrigues, GSOC student.

This new messaging system replaces all the various email_to_user() calls.
They are now replaced by events triggers, and the messages are then
processed centrally according to user preferences and sent to one or more
processors (email, popup, jabber etc...)

This code is not finished yet, a lot of work still has to be done on the
interface.  However, the basic structure is there and should be working.
Luis and I will be reviewing and polishing this heavily in the next few weeks.