Added new gsc function: setgroundreferenceent()

WARNING: I couldn't test this code (nor could I compile it) because fresh OS.

The ground entity's rotation will be added onto the player's view. 
In particular, this will cause the player's yaw to rotate around the 
entity's z-axis instead of the world z-axis. You only need to call 
this function once. After that, any rotation that the reference entity 
undergoes will affect the player. Setting it back to 0 (worldspawn)
should disable all further effects.