L.Ellipse

I've been working with Yuriy Dybskiy (@dybskiy) for the last few days to add Ellipses to Leaflet. He had a client that really needed to be able to draw ellipses (via Leaflet.draw) but since there is no native support for ellipses in Leaflet we needed to add it.

I have also been wondering why there is no support for ellipses which seems like a natural base class for `L.Circle` and `L.CircleMarker`.

This PR adds a new `L.Ellipse` class with a signature of `L.Ellipse(latlng, [radiusX, radiusY], options)`.

I wanted to throw this up and see if its a good candidate for inclusion in the Leaflet core or would be better as a plugin.