Pie plots

Good evening,

Pie plots have been added. Primary projection attributes include value, inner-radius, outer-radius, and fill.
Example usage:

var exampleData = [{name: "Alice", expenses: 20},
                     {name: "Bob", expenses: 30},
                     {name: "Carol", expenses: 50}];
var piePlot = new PiePlot().addDataset(exampleData)
                             .project("value", "expenses")
                             .project("outer-radius", 100);
piePlot.renderTo(svg);

Bugfixes

• Text measurement no longer throws errors in Firefox when the chart is hidden with display: none.
• Scale.Time now properly coerces null values.

API-Breaking Changes

• scale.ticks(count) no longer sets the default number of tick values to generate.
• scale.numTicks(count) has been added, and can be used to set the default number of tick values to generate.