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
Merge pull request #1514 from twpayne/vector-api-circle

[vector-api] Add ol.geom.Circle
Uncomment ol.render.canvas.Immediate#drawCircleGeometry
Merge pull request #1529 from fredj/vector-api-immediate-drawCircleGeometry

Uncomment ol.render.canvas.Immediate#drawCircleGeometry
Add export for ol.geom.LineString#getLength
Add draw interaction events
Add tests for draw interaction events
Merge pull request #1532 from oterral/vector-api-exports

[vector-api] Add export for ol.geom.LineString#getLength
Merge pull request #1524 from oterral/vector-api

[vector-api] Add draw events
Only support square tiles