Allow admins to invite the bot in any channel.

This is a small change that allows the bot to join any channel if invited. If an admin uses it, they may make it join any channel. If a non-admin uses it, they may only make it join the main channel. Added an admin-only !fpart command (both channel and PM). Channel version doesn't require a parameter; bot will simply leave. PM version requires a channel parameter. It will message/notice the person using it if the operation was successful. In either cases it disallows parting from the main channel. It also logs to console invites and fpart uses.