Load function should be called with layer as context

[natevw]

It'd be cleaner to write an ad-hoc layer "subclass" without actually subclassing one if the load function were called with the layer as its this context:

map.add(po.layer(function () {
    console.log(this,arguments);
}));

Currently this logs "Window" as the first object, and the layer is not available to the function unless it is captured in the outer scope.