Allow `drawDots` to be specified on a per series basis

We have a requirement to graphs with a mix of series lines with dots/tooltips and lines without dots/tooltips.

* [x] `drawDots` is an optional param on `series`. Default is true.
* [x] adjusted `getDataPerSeries` method to include param

If `drawDots` is turned off globally at the graph level all dots will be hidden regardless

![image](https://cloud.githubusercontent.com/assets/1502550/3398220/41ff2328-fd2c-11e3-8fe1-a8c2f582e6aa.png)

will update. Good point.