Add configurable GRAPHITE_PREFIX

Would you want the TCP err != nil then nested in that, or would that be redundant?

```
if graphiteServer != "" {
  addr, err := net.LookupHost(graphiteServer)
    if err != nil {
      addr, err := net.ResolveTCPAddr("tcp", graphiteServer)
        if err != nil {
	  go metrics.Graphite(metrics.DefaultRegistry, 10e9, graphitePrefix, addr)
        }
    }
}
```