0.504a - Twitch integration enhancements

Another mid-week build to fix a few issues and improve things. In this one:

• the game now checks every couple minutes to see which Twitch players are currently in the channel; this keeps the game from spamming people if they're not around
• when inviting players back, the invitation sent will include a nice link to your Twitch channel :)
• fixed some bugs that were introduced on the last build - specifically, the auto-camera won't trigger when new players join in a regular game
• added some new Twitch commands - !rotate <coords> lets a player rotate an item or floor, !sell <inventory item> lets a player directly sell items from their inventory

More Twitch commands are coming soon! :)