Hover legend and interactive legend

Added examples of both a hoverLegend and interactiveLegend. The interactiveLegend could possibly replace both toggleLegend and hoverLegend, but at the moment, it feels a bit clunky, since all I did was copy code over to that class. 
i think there's a bug here where if you setup a legend with callbacks, and then set the callback to null, the classes associated with the callbacks will never be reset or cleared off the legend