Improves snappiness of controls
Did a couple minor changes to improve the feeling of the controls. Things should be a lot snappier now.

- Reduces the keypress interval from 0.25 to 0.1
- Allows stopMoving to be called on player even if startMoving is still happening. This results in jerkiness which we need to fix, but helps with the snappiness.
- Adds a TURN_BUFFER constant so that we're not using the keypress constant to test for turns. This can be set longer to allow more turning flexibility (currently set to 1 second)
- Refactors the actual lane change code into the player class as the "moveBy" method.

TODO:
- Fix the jerkiness bug
- Tweak the lane change timing in concert with the turn timing and keypress timing. Lane change still feels a tad sticky
- There's a rotation bug where the rotation will occasionally flip back around or something and cause the ambulance to not be oriented up. It fixes itself eventually sometimes. Need to figure out the cause.