Only offer actual mailboxes in bic--read-mailbox

Filter out non-directories for completion.  That ensures that we don't
offer "connection-info" as a mailbox.
Store mailbox state in hash table

Currently, "mailbox state" includes only existence.

Also, add hooks such that the mailbox tree can be updated
automatically.
Display mailbox tree buffer at start

This should be the most convenient and intuitive interface.
Strip trailing newline from sentinel message in :connecting state
Add README.org file