Combine multiple messages with "\n".

Multiple messages sent to statsd must be combined with `"\n"`. In Ruby, `'\n'` is the same as `"\\n"`.

Without this patch, I experimented the following error:

```
2014-09-09 11:07:55,278 | ERROR | dd.dogstatsd | dogstatsd(dogstatsd.py:253) | Error receiving datagram
Traceback (most recent call last):
  File "/usr/share/datadog/agent/dogstatsd.py", line 244, in start
    aggregator_submit(socket_recv(buffer_size))
  File "/usr/share/datadog/agent/aggregator.py", line 374, in submit_packets
    self.submit_metric(name, value, mtype, tags=tags, sample_rate=sample_rate)
  File "/usr/share/datadog/agent/aggregator.py", line 384, in submit_metric
    metric_class = self.metric_type_to_class[mtype]
KeyError: 'h\\nmetric.name:12345'
```