Implement asynchronous chat support.

This is like the support from Bukkit, which dictates asynchronous chat only for chat messages from the network.

This is my first contribution to Glowstone, so please don't bite hard.
It is strange, but in order to comply with the Bukkit API which mandates chat() to be synchronous and network chat to be asynchronous, this is the way I have to do it.

But since you did bring up the brackets, I'll add those.