Doors should play closing/opening sounds

[robro]

The opening sound is a single file but to get the closing and slamming sounds to line up with the animation they probably need to be played individually. The closing sounds should be cut-off when the slamming sounds play.

The gateClose and doorsClose sounds should be triggered at the start of the animation. gateClose should be cut-off by the wireGate sound while doorsClose should be cut-off by the steelGate sound.

[robro]

Mock-up of the transition with the new sounds: https://vimeo.com/55649543

It actually sounds best if the sounds begin slightly before we see any part of the transition, which would be easy to implement by just adding a couple blank frames to beginning of the animation (there's blank space at the end of the sprite sheet).