Fix: failing on non-existent IMAP mailboxes.

Fixed bug where script would fatally terminate on a non-existent
mailbox. This also creates nested labels for Gmail. Based on this
patch:

http://sourceforge.net/p/imap-upload/bugs/7/
Added documentation.

Added features and quick start documentation copied from original IMAP
Upload web site:

http://imap-upload.sourceforge.net/