Create the PlayerLog class, and log stuff to it

We now log player connections, disconnections, warp creations and
teleportations. It's scalable enough to add loads of things to it when
we need it.
Move DatabaseThread to its own file

I'm getting weird NoClassDefFoundError exceptions when reloading the
plugin after I recompiled it. Let's move out DatabaseThread to its own
file to see if that works, since it's the only class which is giving the
error for me.
Implement support for /warp list.

This command allows a player to list the warps which they created in the
current world.