[vector-api] Add ol.geom.Circle

This PR is opened for discussion.

We often need to draw circles. This PR draws circles in the projected coordinate system.

It will confuse people who think that Spherical Mercator (EPSG:900913) is a good projection. Circles in Spherical Mercator bear no resemblance to circles in the real world, unless your circles are small and either you're near the equator or you compensate with point resolution.

GeoJSON doesn't have circles. Trying to serialize an `ol.geom.Circle` will currently cause assertion failures (if you're running a debug build of ol3) or just weird errors (if you're using a built version of ol3).

So... this is definitely useful functionality. The only question is how best to implement it.