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. 
maybe this could have a more descriptive name? also, if this is being attached to `this.content`, will it ever have a datum argument to the callback? maybe it should just be a function without any arguments, since seeing it take a datum is a little confusing