added in dataseriesindex when drawing custom symbols, useful for using d...

This change is to allow us to change the point symbol for points in the same series.

For example:
```javascript
var timeline_data = [ [x1 , y1] , [x2, y2] , ... ]
var timeline_labels [ {label: "label1" , type: "redsquare"} , {label: "label2" , type: "yellowcircle"} , ... ]
```
Would allow you to have a custom label and icon for each datapoint referencing each other through their index.

![flot-timeline-custom-symbol](https://f.cloud.github.com/assets/1423728/1156519/4db215bc-1f89-11e3-90f3-f4f9a1a80c25.png)