Fix full circle pie slice (100%) drawing

Having a single hostgroup, and all hosts and services up, landing page Pie Chart is empty. This patch set restores display of OK hosts and services. As a follow up, getPieSlicePath primitive can draw nice looking full circle pie slice.

![piechart_circle](https://cloud.githubusercontent.com/assets/712022/3990937/0450ca28-28d7-11e4-8da9-0db84093961c.png)