Graph

This adds a new D3GL data type: d3.gl.graph. D3GL graph supports 3D scatterplots, line graphs, axes, labels, and ticks. The demos iris.html, neurons.html, and pointcloud.html use d3.gl.graph.

Also there was a structural change so that d3.gl.globe, d3.gl.model, and d3.gl.graph share as much code as possible.