Add gamestate.extend() for convenience
Rename gamestate.onEnter and gamestate.onLeave

These methods are now called gamestate.enter and gamestate.leave for
consistency.

This is part of #9.
Rename statemachine.findBaseState

The replacement is statemachine.findbase.

Part of #9.
Rename camera.panCenter

The replacement is called camera.pan.

Part of #9.
Replace camera.calculatedX() and camera.calculatedY()

A new function, camera.position() is introduced. It performs the functions
of both the removed functions.

This is part of #9.