Rudy/multiple metrics

Allow to send multiple metrics in the same request.

The syntax is as follows:

```
require 'dogapi'
dog = Dogapi::Client.new('api_key')

dog.batch_metrics do
    dog.emit_point('my.metric.a', 50)
    dog.emit_point('my.metric.b', 59.9)
end
```

All the metrics inside the block will be sent in a single request to the server.

Fixes  #22