added path preview and esc-key undo to BezierTool

Added some features to the BezierTool example:
- path preview: line follows mouse (onMouseMove) to show what would happen if you placed a point -- accomplished with the placeholder nextPoint variable
- undo: onKeyDown handles escape key to remove the last point added to path

Learning how this library works and also trying to make this example act more like Illustrator pen tool. Let me know what you think.