Added basic functionality for image nodes

Extended the drawNode function to include basic image drawing. Adjusted the height and width functions to accommodate natural image dimensions as well as those set with node.data.image.width etc.
License mistake

Project standard doesn't seem to list all contributors in the license, so removed myself
Added auto text orientation for edge labels

Added new boolean option edgeLabelsUpright which makes edge labels automatically flip over the edge when they are upside-down.  Addresses readability aspect of #56.