Problem: mlm_client_new does not send address

This is needed to set the 'sender' in deliveries, and for mailbox
routing.

Solution: add address argument to constructor and expand test cases
to check this is properly sent in deliveries.
Problem: need MAILBOX functionality

Solution: implement minimal mailbox functionality and MAILBOX SEND
and MAILBOX DELIVER commands. Still to be fully tested.

Note: mailboxes are asynchronous (client can fetch mail at any time)
however they are not persisted to disk, and messages are not tracked
at present (so are lost if client dies while processing them).
Merge pull request #30 from hintjens/master

Adding request-reply support