Added in public properties to give legend buttons rounded corners

Also added in a rounded_rect path creating function to create specific rounded corners.

eg. 
var chart = nv.models.stackedAreaChart()...

chart.legend.svgComputedTextPadding(40); // adds padding between buttons
chart.legend.roundedRecRadius(4); // sets a rounded radius
chart.legend.legendBelowPadding(20); // adds padding between legend and chart