Added a way to set the onclick function for a parent label.
Cleaned up some of the code.
Fixed an SVG width issue where if no data was supplied, the width and height were set to -15, which threw an exception.
Fixed a bug where if the tooltip width and height got too big, the arrow wasn't pointing at the child node.
Fixed a bug where the width of the SVG was being determined incorrectly.
Added a way to not show the value on the tooltip by setting the valueKeyName to an empty string.
Added a cursor pointer if there is an onClick function.
Rewrote the source in ES6 and d3 v4.1.0