Remove compiled code.
Simplify movie script and make it compatible with both
OSX and Linux with avconv.
Add collision test code and use N = 2 particles.
Ignore some generated files.
Add notes about generated movies, and changes to code.
Improve layout and simplify code, no functional changes.
Add functions for linear/angular momentum of a set of particles.
Simplify calculation of F_{1,2} with Taylor series;
there should be no functional changes.