LineMode: 'dashed' will render a series as a dashed line

We have a requirement to render series line as a dashed line, in our case, to display the 'averages' and the 'standard deviations'. We discovered by adjusting a SVG style property this was possible.

The style `stroke-dasharray` can be used to draw a dotted line. See https://leanpub.com/D3-Tips-and-Tricks/read for more details.

The property can be added to a `series` like so:
```
{y: 'value', color: 'steelblue', thickness: '2px', type: 'line', lineMode: 'dashed'}
```

* [x] updated `getDataPerSeries` to include the 'dashed' property
* [x] `lines.coffee` now tests for the property as acts accordingly.
* [x] updated tests to include new style

![image](https://cloud.githubusercontent.com/assets/1502550/3370094/afa3d872-fb8a-11e3-8c95-ed1a0fa66ada.png)