Allow the donut to be a pie chart as well

Adds some attributes for the inner-radius and label-radius which are
relative to the total radius (0..1 scale).
Allows pie chart to be themeable with CSS

Added a default theme file and integer based classes for the arcs so
they can be individually styled.
Merge pull request #10 from WealthBar/pie-chart-improvements

Improvements for pie/donut charts