Use the go-graphing library for bstat

The d3 and rickshaw styles/js have been relocated to an open source `go-graphing` plugin.  Submodule that repository and leverage the singleton.  See the [README of go-graphing](https://github.com/GigaOM/go-graphing/blob/master/README.md) for more info.

See: https://github.com/GigaOM/legacy-pro/issues/3619