Add support for more complex search expressions.

This library is exactly what I needed except for the limited searching. I needed to be able to search for messages sent to a specific address, with a specific subject and optionally with certain text contain in the body.

I took the liberty of implementing a more advanced search expression feature utilizing each of the search keywords supported by IMAP2. IMAP4 introduces more advanced searches but would require the use of sockets as the PHP extension only supports IMAP2.