now more smoooooooth and faster!

fixed deceleration as well as some other errors with velocity
calculations. now it seems to be working very well with the turn and
go motion primitives!
reset velocity after a turn in place.

this means the robot accelerates away from the turn in place
instead of just instantly jumping to max velocity. oops.
go faster!

watch the heck out! 1.4 meters per second is no joke!
the robot can follow paths this fast without appearing to slip the
wheels, but it also looks scary and probably goes too fast for the
slam code.