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.
Merge pull request #1 from Norkart/master

Added BSD License
Update README for Shove Gesture
Merge pull request #2 from Norkart/shove

Implements a two-finger shove gesture
Added Shove Gesture to example activity

Doing a two-finger shove will increase or decrease the alpha value of
the planet, rendering it more or less transparent.
Merge pull request #3 from Norkart/shoveExample

Added Shove Gesture to example activity