Refactoring Springs

[Hounshell]

Hey, I'm using p2 for a display I'm building, but I'm trying to model force interactions that aren't accurately described by springs. I think this refactoring abstracts forces a little differently, and it helps form the basis of other attractors/repulsors. I've run the tests and nothing seems broken. Not sure what else you'd like me to run to verify that changes are non-breaking.

[pkieltyka]

this looks very cool, any chance to get this reviewed/merged?