K nearest neighbor for objects

var k = require('./lib/index');

var center = { a: 1, b: 2 },
neighbor = { a: 1, c: 4 };

k.neighbor( center, neighbor ); // return 2 for distance from center to neighbor

neighbor = { a: 1, b: 2 };

k.neighbor( center, neighbor ); // return 0 for distance from center to neighbor