Strafing

You can toggle strafing by pressing 'U'.  Current implementation has it
so that if you are moving forward while strafing, it is normal speed,
while other directions have reduced speed.  This can be easily changed.

For this to work, your movement direction and the direction you face had
to become separate variables.  If you aren't strafing, you will always
face the same way as you move, unless explicitly set every single time.

Misc:  Made grave shake when it appears.