First pass at ShoveGestureDetector

It works for reporting shoves but does not discriminate where your
fingers are shoving, so it will happily report shoves during both
rotations and scales, as well as when your fingers are above each other
instead of beside each other.
Check that shove gesture is proper

Now shove actions are only initiated if the fingers are within 20
degrees of each other.
Update README for Shove Gesture
Merge pull request #2 from Norkart/shove

Implements a two-finger shove gesture