Add a strokeColor options to the Donut class

Add a new option to the Donut class to be able to put the donut on another background than a white one.

(and update the package grunt-less as suggested in the README
https://github.com/jharding/grunt-less/blob/master/README.md#grunt-less-deprecated)

![Capture d e cran 2013-01-18 a 01 15 17](https://f.cloud.github.com/assets/328718/76851/3010f4c2-6104-11e2-87ed-d6035626d2f6.png)