Feature fix piecharts

Fixes https://github.com/theodi/dashboards/issues/120

After much grappling with D3, piecharts now look like this:

![pie1](https://cloud.githubusercontent.com/assets/109774/2615654/ba8fb3dc-bbfc-11e3-94e4-a9d8e59dad11.png)

![pie2](https://cloud.githubusercontent.com/assets/109774/2615656/bc6852b8-bbfc-11e3-8e8a-c079750367a5.png)