Adding a HTTP writer for Graphite

Hi,

The aim of this pull request is to pass through an HTTP Proxy to reach a graphite installed behind such a proxy.

The graphite behind the proxy needs to transform the HTTP POST sent by this new writer into a classic socket (see transformation done by a python proxy here : https://answers.launchpad.net/graphite/+question/213436)

I don't know if this specific need can help anyone else, but if so, here it is.

Simon