Example labels

Add a new example demonstrating what's possible to do with labels in OL3.  It includes 3 geojson dataset samples of points, lines and polygons.  In addition, 3 forms allow the user to change text label styling on the-fly and see the result.

See the [demo in action](http://dev5.mapgears.com/bis-ol3-proto-label/examples/vector-labels.html) .  See also an [OpenLayers 2.12 equivalent demo](http://dev5.mapgears.com/bis-ol2-proto-label/).

Comments would be quite welcome.