Graphite handler unittest + batch join bugfix

Thanks so much for the backlog functionality (#253). It was on my wishlist.

To check/test it, I have added a unittest for the GraphiteHandler.

I also spotted a small bug when sending in batches. Current code does a:
```
self.socket.sendall("\n".join(self.metrics))
```
... where self.metrics as metric strings. But these metric strings come from Metric.__repr__ which already adds a newline. So I have replaced the "\n".join(...) by a ""'.join(...).

The new unittests verify this bugfix and backlog functionality.

Regards.