Add optional IPv6 support

Added a 3rd argument to the StatsD initializer, a boolean which we use to specify if our host is an IPv6 address or not.

I chose to strictly specify the type since we don't want to resolve the host to determine if it's an v4 or v6 (and trust me, OS X will damage your brain when you get into that.. something about the default `/etc/hosts` having multiple addresses resolving for `localhost`, and Ruby choosing `::1` as the default for `IPSocket.getaddress('localhost')`.. what a pain.. ANYWAYS.

I'm well aware the StatsD daemon doesn't listen on IPv6 **yet**, but it doesn't mean being able to send stats to an IPv6 address shouldn't be possible ;)

Here's a first attempt at enabling it. :ocean: 

Usage:

```
$statsd = Statsd.new '::1', 9125, true
```