Give features unique identifiers

With 652f11cefa5352eabc63eab3afe7349c6aad057c vector features can no longer have duplicate identifiers (see #2087).  This breaks the vector layer example (in [dev mode](http://ol3js.org/en/master/examples/vector-layer.html?mode=raw)) because Northern Cyprus and Somaliland have the same `id` in `countries.geojson`.

We could remove the assertion and allow duplicate ids.  Thoughts?

In either case, I think it makes sense to give them unique ids in this dataset.