Continuous updating of PNCircleChart

In order to update one PNCircleChart which is made previously,
gradeLabel has to be private property of PNCircleChart.
Bacause I don't want previous gradeLabel to be kept on the UIView for the above purpose,
I wrote this patch. Please consider this specification.
Merge pull request #28 from Lewuathe/update-circlechart

Continuous updating of PNCircleChart