Canvas Renderer

1. Adds a canvas renderer fallback to ```threejs-layer.js```. For this, there's a new ```webgl``` property that basically mimics the one in ```detector.js```. If no webgl context is detected, the lib will instantiate a THREE.CanvasRenderer object.
2. Fixes the example accordingly. The fallback renderer can be forced by adding ```renderertype:'Canvas'``` to the constructor
```js
new ThreejsLayer({ map: map,renderertype:'Canvas' }, function(layer){
...
});
```
3. Replaces the version of ```three.js``` in ```/lib``` for the full r68 version. For some reason, the version you had didn't had the ```THREE.SpriteMaterial``` object.