New `drawScrubber` series option to allow scrubber to be show on a per series basis

In a similar vein to `drawDots` that allows you to tune a graph on a per series basis this will allow you to turn on/off the tooltip scrubber on a per series basis.

* [x] new `drawScrubber` option on series config

![image](https://cloud.githubusercontent.com/assets/1502550/3658065/8ab1b73a-119e-11e4-87a8-a4de8dd7cf9f.png)