`Map`: `add_circle()`

[clarisma]

Allow users to directly add circles to a Map.
Right now, the only way to show a circle is buffering a point to create an approximate Shapely Polygon:

circle = shapely.buffer(point, to_mercator(meters=100, lat=38))
map.add(circle)

This would be nicer:

map.add_circle(point, meters=100)