Prevent multiple renderings of the background rect when updating chart
Allow piePadding to be an object with top, left, bottom, right values to have better control on pie positioning also allow padding and radius to be % values

Fixes #142, #143
Change positioning of pie charts to be centered by default

The pie will be centered vertically in the container unless a padding top or bottom is specified,
The pie will be centered horizontally in the container unless a padding left or right is specified

Fixed #144