Add ol.css to examples

Currently the examples do not include ol.css. See http://camptocamp.github.io/ngeo/master/simple.html for example. As you can see the zoom and attribution controls are not displayed on the map. This PR fixes that. See http://erilem.net/ngeo/css/simple.html.

Note: at this point `ngeo.css` just include the ol css. In the future ngeo will probably have its own css, so `ngeo.css` will include both the ol css and the ngeo css. We will also need to decide whether we use a CSS pre-processor like Less.

Please review.