Rudy/batch metrics

Allow to send multiple metrics in a single udp packet.
To send them in batch, use 'BatchedDataDogstatsD' instead of `DatadogStatsd` and add a call to flush_buffer at the end.

To avoid to create too large UDP packets, if more than 50 metrics are reported, it will automatically be flushed.

Similar to https://github.com/DataDog/dogstatsd-ruby/pull/7 and https://github.com/DataDog/dogstatsd-python/pull/19