Negative values cannot be represented on a pie chart.
Merge pull request #46 from benpickles/negative-pie

Negative values cannot be represented on a pie chart
No need to calculate the dimensions from the SVG element.

This was a throwback to making <canvas> crisp on variable pixel density screens.
Utilise the jQuery object.
Don’t blow up if a pie/donut’s sum is less than zero.
Merge pull request #47 from benpickles/zero-sum-pies

Don’t blow up if a pie/donut’s sum is less than zero.
Expose #svgElement method to the outside world.
Add an after hook - called on every draw - that might be used to decorate the chart.