Custom point size

[eastbayjake]

Adds a changePointSize method so users can specify a custom point size. (It currently defaults to 2.) Example usage:

// define mapCanvas and mapOptions as in demo
map = new google.maps.Map(mapCanvas, mapOptions);
var myLayer = new WebGLLayer(map);
myLayer.changePointSize(4);
myLayer.start();

Closes #3 (cc: @korczis)