Add example combining draw and modify interaction

This PR adds an example that demonstrates how the draw interaction can be used together with the modify interaction using an `ol.FeatureOverlay` (ticket https://github.com/openlayers/ol3/issues/2104).

A little issue exists when the geometry type of the draw interaction is set to points. When hovering existing geometries, you will see two points: the point of the draw interaction and also the sketching point of the modify interaction. The interactions work as expected, it is just that two points are shown. Suggestions are welcome on how to address this!

Demo: http://tsauerwein.github.io/ol3/draw-and-modify/examples/draw-and-modify-features.html