Add atlas manager

 * Introduces a `hashCode` function for `ol.style.Circle`
 * Adds an atlas manager that uses the packing algorithm described in  http://www.blackpawn.com/texts/lightmaps/

Currently still missing:
 * The hash value is not cached.
 * When the atlas manager creates a new atlas, it uses the same size (default: 256x256px). We could double the size every time we create a new atlas.