Add PolarCoordinates task

Cartesian <-> polar coordinate transformation

One can turn this
![cartesian](https://cloud.githubusercontent.com/assets/2648829/4827914/f714c16c-5f7b-11e4-980d-b23bff350caf.png)

into this
![polar](https://cloud.githubusercontent.com/assets/2648829/4827915/fad8c442-5f7b-11e4-9d5a-7012906c83cf.png)

and back