Back of track now extendable with a variable

The back of the track will now extend backwards from the position of the
player a distance denoted as backLength that is specified in the
GenerateRails script. The code for moving the rails should be more
optimized now, as it only moves the rails it needs to move (the back
rail to the front). In fact, in it's present state, no rails are moving
because the track isn't changing....and we aren't really moving...things
to think about.
Moved source of gunshot sound from bullet to gun

Solves a clipped audio problem when clown is murdered close to cart. And
besides, guns make the bang sound. Bullets don't make the bang sound.
TimeScoreUpdater moved and fixed

Fixed TimeScoreUpdater and fixed problem of the timer still running when
the game was paused.