Add @api to ol.Sphere

Adding `@api` to `ol.Sphere` makes it appear in the API docs, and provides a link from `ol.geom.Polygon#circular`. As commented in https://github.com/openlayers/ol3/issues/2359 this does not help with `ol.Sphere.WGS84`, but that problem can be handled separately.

Adding `@api` to `ol.Sphere` also fixes a problem in the externs file generated by the `generate-externs.js` task. Without the `@api` annotation the externs file includes references to the `ol.Sphere`, which is unknown.

Please review.