Add ol.geom.Circle
Add ol.render.IRender#drawCircleGeometry
Add ol.render.webgl.Immediate#drawCircleGeometry
Add ol.render.canvas.Immediate#drawCircleGeometry
Add ol.replay.canvas.Instruction.CIRCLE
Add ol.render.canvas.PolygonReplay#drawCircleGeometry
add ol.render.vector.renderCircleGeometry_
Add an ol.geom.Circle to geojson example
Add FIXME to geolocation example
Encode ol.geom.Circles as empty geometry collections in GeoJSON

GeoJSON does not support circles, nor null geometries.  Empty geometry
collections seem to be the way to represent at null geometry in GeoJSON.
More efficient ol.geom.Circle#closestPointXY, thanks @tschaub