Fix Jumping

There seems to be a bug around constantly accelerating the player into a
surface in the current version of HaxeFlixel. I've made a note to
investigate this further after EXPER3, but in the meantime, separating
the player from the floor by a few pixels before jumping does the trick.
Tune Player Movement

Finesses the player movement towards something that feels better to
control. Horizontal movement no longer uses physically-accurate
acceleration, because it felt too mushy.

Also, this contains workarounds for more HaxeFlixel physics bugs. I'm
getting the impression most people use HaxeFlixel with a third-party
physics library, so the built-in physics aren't well-tested.