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.
Remove this file. It's not used.