Vislib: Area Chart

Adds area chart to vislib.

Area chart has several modes which can be specified in the `params` passed to the vislib Vis constructor.

The modes are as follows:

1. stacked (default) - stacked area chart
2. overlap - overlapping area chart
3. percentage - percent area chart
4. wiggle - minimize weighted change in slope.
5. silhouette - center the stream, as in [ThemeRiver](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.39.2977&rep=rep1&type=pdf)

For example:

```javascript
var params = {
  type: 'area',
  mode: 'percentage'
};

var myChart = new Vis($el, params);
```

Stacked
![screen shot 2014-10-15 at 3 42 40 am](https://cloud.githubusercontent.com/assets/3149785/4639052/31764330-5405-11e4-8393-b0d4def2a9a8.png)

Overlap
![screen shot 2014-10-15 at 3 43 00 am](https://cloud.githubusercontent.com/assets/3149785/4639054/393d07fc-5405-11e4-81ca-51de25d830e6.png)

Percentage
![screen shot 2014-10-15 at 3 43 24 am](https://cloud.githubusercontent.com/assets/3149785/4639057/433b5a38-5405-11e4-9d09-67d20f49e3c5.png)

Wiggle
![screen shot 2014-10-15 at 3 43 45 am](https://cloud.githubusercontent.com/assets/3149785/4639058/48f89aa8-5405-11e4-82cf-9c9e45ebaa05.png)

Silhouette
![screen shot 2014-10-15 at 3 44 07 am](https://cloud.githubusercontent.com/assets/3149785/4639060/5caa1bb2-5405-11e4-8e77-fb56f8c24428.png)