Added support to define a color per bar.

Implemented the array property „strokeColors“ in order to define a custom color for each bar. If the „strokeColors“ count does not equal the „yValues“ count, the „strokeColor“ is used as the fallback color.
![Barchart with custom bar colors](https://f.cloud.github.com/assets/975938/1797728/1180a3c4-6b06-11e3-8421-b9c4ae11685d.png)