PieChart labels don't support extended format string syntax

[sykoram]

Steps to reproduce

1. Define custom class inheriting from PieSlice with some custom property to use in the labels (eg. PropertyX)
2. Create PieSeries where the InsideLabelFormat or OutsideLabelFormat includes the custom property ("... {PropertyX} ...")
3. Try to render the plot

Expected behaviour

The plot is successfully rendered with the labels containing the values of the custom property.

Actual behaviour

An exception is thrown.

Fix (?)

I'm not familiar with the whole OxyPlot's code, but looking at the PieSeries class, the fix might be really easy: just replacing two string.Format statements in Render method with StringHelper.Format. I can create a pull request if all it takes is really just these small changes.