Enhancements:
* Squashed all commits prior to these updates to remove huge legacy files
* Works across mac, linux, windows 10. Only .svg output on linux, windows
* Leaf nodes respect class weights now in classifier trees, thanks to dschoenleber
* SVG files on all platforms embed images for nodes, rather than linking to them. We had to do this with custom XML munging code, except on OS X where we can do `-Tsvg:cairo` on `dot` command
* Legend on classifiers now done with mathplotlib not graphviz