Added a way to compute a "histogram" (ordered by time slices).

This allows us to see snapshots of the online statistics algorihm.
Warn instead of throwing an exception on packet type change.

We don't want to crash plugins over this, since it doesn't 
automatically cause problems. But it may trip up plugins that 
assume the packet types they set when registering a listener is the 
only ones they'll ever recieve in the method body, which is not true
if a preceeding packet listener can change a packet to an arbitrary 
type.

I'm open for better suggestions here. But for now, I'll just print a 
warning and hope people use sendServerPacket() instead.