Support for multi colored bars in bar graphs

Added support for multi colored bars in bargraphs (fillStyle and strokeStyle).

`fillColor` and `strokeColor` in the `datasets` definition can now be arrays of colors. Each bar in the bar graph should then cycle through these colours.

Here's the modifed example from bar.html and what it looks like:
```javscript
fillColor : ["rgba(220,220,220,0.5)", "rgba(235,136,0,0.5)"],
strokeColor : ["rgba(220,220,220,1)", "rgba(235,136,0, 1)"],
````

![image](https://f.cloud.github.com/assets/547147/643683/51d948c2-d37b-11e2-872e-6029573aad24.png)