VMM/Account: Adjusted to changes in maillocation.MailLocation.
VMM/maillocation: Class MailLocation added missing property `mid`.
Added new error to VMM/constants/ERROR and doc/source/vmm_constants_error
VMM/Transport: reworked Transport class.
Use assertions for argument checks. Removed methods getID() and
getTransport(). This values are now accessible through the read-only
attributes id and transport.
VMM/{Account,Domain}: adjusted to modifications in Transport class.
A few code cleanups.