add Graphite formatters

Hi,

This PR allow to use Phantomas to easily export results into [Graphite backend](http://graphite.wikidot.com/).

```
./phantomas --url http://www.google.com --format graphite | nc mygraphite.org 2003
```
will generate a graphite node : `stats_counts.phantomas.google-com.<metricsname>` for each metrics computed by Phantomas.

You could then add a cron job for your website to do this every 10 minutes for exemple, in order to have an impressive dashboard for Synthetic User Monitoring made by Phantomas, like this :

![webperf_phantomas_-_graphite_dashboard](https://f.cloud.github.com/assets/1484406/1850995/6f2f5d9a-76e0-11e3-9914-527e062b1597.png)

Thanks.