Add graphite, scalable realtime graphing service

This one depends on offlinehacker/nixpkgs@50021ebb0a94d4dec5752dfea3e58b02c55f16af

It could be better in some aspects: 
- Support for additional config for graphite_web, beause the django "magic" does not help here, but that's not so important, we usually leave this config as it is anyway.
- Use gunicorn or uWSGI for serving, but we don't have those two packages, so i just use django development server, but again not that important for me.

**The summary:** carbon, the graphite database, can be very nicely configured, but again, yes, django is a mess.