Move code out to .js files
Add "vector" functions

Added some preliminary functions to get the basic directions down.
This isn't actually a potential field at all, come to think of it, since
the boids aren't moving toward the highest potential, but rather
repellant fields, which push the boids away. What I'm going for is a
topographical map where boids go "downhill".
WIP: whip up a test framework to run on body load

I don't really want to mess around with a JavaScript unit test framework
right now. Each .js file will collect its own tests and then the
boids.html file will collect all the tests, run them and then print out
any failures. This way the tests are run every time the page loads.
WIP: more work on testing
Test framework in place; test not passing