Add API to control whether a player affects spawning

This is a patch by @Jedediah from [SportBukkit](/OvercastNetwork/SportBukkit). It adds methods which give plugins the ability to decide whether or not a player will affect spawning. (I.e. activate spawners when in range, and contribute to other mob spawn calculations).

This is useful to plugins which provide spectator functionality, such as [AutoReferee](/rmct/AutoReferee), because it allows them to eliminate one way in which spectators can accidentally interfere with gameplay.