Changed the clock

As it stands the clock can be a bit confusing to use. elapsedTime only returns a variable if you call getElapsedTime() or getDelta() on each frame, and if you need to use both you must call getDelta() before getElapsedTime() otherwise getDelta() will be incorrect (or unexpected depending on how you look at it). Also changed the constructor so that autoStart automatically starts the clock straight away (which would be the expected behaviour), changed getDelta() to getDeltaTime() for more consistent naming between the time getters and removed elapsedTime since it is now redundant