Complete refactor of API

 * Front factory is now defined as an Index instance instead of the
   MailReaderInterface service;

 * MailReaderInterface object is an internal property of Index;

 * Moved some things arround, see commit diff;

 * Refactored the various Model\ implementations in order to factorize
   as much as code possible, and expose simpler concepters for the
   end user;

Warning: code is not usable and is expected to crash at this stade.