Switched to verlet integrator

Also player inputs now apply force instead of setting velocity. This
should make things easier to tune. Will need to apply a mitigating
damper of some sort to prevent runaway condition.
Changed player object in test to a vector of players. Hopefully will
figure out how to conveniently add more players without getting too
messy.
Removed a bunch of debug output from player class, if I add them back
they will go into test - the player class will simply output numbers
only.