Show legend to the right of the pie chart if width > height

pieChart currently shows the legend above the graph. For vertically constrained pages, having the legend to the right of the graph would be beneficial. This patch shows the legend to the right of the pie chart for charts in landscape mode (i.e., wider than tall). It has been tested to the best of my ability not to impact the existing demos. Passed grunt testing.

This pull request replaces the one I accidentally sent to liquidpele/nvd3 and closed myself.