Added the new Grafana dashboard  to tools.rst

A simple pull requests that adds my new dashboard [Grafana](/torkelo/grafana) to the tools list. 

Unlike most alternative dashboards that requires json file editing, this dashboards places a heavy focus on making editing of dashboards and graphs easy. Includes a graphite target expression parser than enables easy metric and function editing. It is based on the great log dashbord Kibana but heavily modified and stripped down to be a great Graphite dashboard and graph composer. It uses the new maxDataPoints parameter to make client side rendering fast even over large time ranges. Visit [grafana.org](http://grafana.org) for more info.