Add a layer catalog example

This PR adds a layer catalog example.

See http://camptocamp.github.io/ngeo/catalog/layer-catalog.html for a live example.

The layer catalog is a layer tree with checkboxes to add/remove layers to/from the map. This shows how to implement recursive directives. It also shows how to use Angular's one-time binding functionality, which is important for the performance.

It may make sense to add a catalog directive to the core of ngeo in the future, but I'd first like to secure this work by creating an ngeo example.

Please review.