Add time window to prevent repeated away messages

[xavid@mit.edu]

On AIM at least, it's reasonably common to hold conversations with people who claim to be away (but aren't really). This is rather annoying with Barnowl, since the aaway message is sent in reply to every incoming message. Probably the easiest way to avoid this would be to not send an away message to a given recipient if a message has been received from or sent to them in the last few minutes, or if the away state has changed since last contact.

This probably wants to wait for #84.