Added new player-speed-max section to bukkit.yml to control speed limits.

Some mods, such as Dartcraft, add items to the game that allow players to move
at abnormal speeds. This can cause many performance issues on servers
since an excess amount of chunks get loaded as players move across land at
these fast speeds. To solve this, I have added the first limit for land
movement. Tested with Dartcraft Armor and was able to limit it properly.