Metrics batching

Similar to https://github.com/DataDog/dogstatsd-python/pull/19

Allows batching of metrics in a block:

````ruby
statsd = Statsd.new('localhost', 8125)
statsd.batch do
  statsd.gauge('mygauge',5)
  statsd.gauge('myothergauge',10)
end
````
will send only one udp packets containing both metrics

'metrics' => 'metrics'

Also, be clear about when it's being sent - when the block completes?