Add color option to BoxHelper constructor

This simple PR adds a color option to BoxHelper's constructor.  Valid arguments could be, for example:

var helper = new THREE.BoxHelper(box, 0xffffff);
var helper = new THREE.BoxHelper(box, 'rgb(0,255,0)');
var helper = new THREE.BoxHelper(box, 'red');

If no color is specified, it defaults to a Yellow color, which is the current three.js color default for BoxHelper.