Bugfixes and Text

- Rover exploding sound now only happens when the rover explodes
- Fast Forward button should now properly effect the score when running,
but I haven't formally tested this yet
- Working on better repositioning code to keep the rover on the grid.
Right now fast forward and changing direction moves it slightly off the
grid which can potentially lead to some hitbox issues.
-Added level-specific mission text to help the player easily understand
obstacles for the first three missions.