README: added 'uid' to example of message iteration
imbox: call close() before logout

The imaplib documentation recommends calling close() before logout to ensure
that 'deleted messages are removed from writable mailbox.'
imbox: moved attribute creation to __init__

This makes pylint happier :)
imbox: added folders() method to Imbox

I think it would be nice to have the list of folders on the Imbox class along
with the message query rather than on the ImapTransport class. However, I have
left the existing list_folders() method on the ImapTransport to avoid breaking
existing code that is using imbox.
Merge pull request #25 from jdowner/minor-fixes

Minor changes
Merge pull request #24 from jdowner/readme

README: added 'uid' to example of message iteration