Implemented auto scaling of DonutChart

[thomai-d]

Hi Alois,

thanks for your awesome, simple and well written charting lib.

I used the DonutChart on a mobile device and ran into the same troubles as #85.

So I extended your code to automatically calculate the drawable chart area - at least for the DonutChart:

For future debugging I thought it might be useful to leave my debugging rectangles in the code - activatable by setting Chart.DrawDebugRectangles.

Also I found it useful to turn all the labels for the DonutChart to the right side to display a bigger chart on mobile devices in portrait form (DonutChart.CaptionOnTheRight)

This is my first pull request on github. If I missed something, please let me know.

Thomas

[thomai-d]

I see this branch also covers #65