Addition of method to retrieve ping from a player. Adds BUKKIT-2437

Allows the user to retrieve the ping of a player.

Why is this important?

Wikipedia.
>Having a low ping is always desirable because lower latency provides smoother gameplay by allowing faster updates of game data.
>Ping is often conflated with lag. One may "lag out" due to unacceptably high ping. Servers will often disconnect a client if the ping is too high and it poses a detriment to others' gameplay. Similarly, client software will often mandate disconnection if the ping is too high. A high ping is not the result of lag; rather, a high ping causes lag. It may also make servers crash because of the stability.