Rename newton/newton.go -> newton/algorithm.go
Add newton/doc.go
Add newton.System
Fix README
Added .gitignore
Add newton.System.{Set,Add}Force
Add newton.System.Step
Add newton.*Body.SetMass
Add newton.*System.{Bodies,Body}
Use newton.System for the Euler-based simulation in shm
Use newton.System for the Verlet-based simulation in shm