Changed CVT behaviour, far less annoying sound

CVT now has a target rpm range, instead of a single rpm.  This lets the
engine rpm vary with speed, so you don't have to listen to a single
monotonous tone for 5 minutes straight.  Also results in a slightly
faster response time.
Merge pull request #23 from Fervidusletum/CVT

Changed CVT behaviour, far less annoying sound