Added composer support

Converted to a composer package. The package can be added to a project by adding
```
    "repositories": [
        {
            "url": "git@github.com:iFixit/statsd-php-client.git",
            "type": "git",
            "reference": "master"
        }
    ],
    "require": {
            ...
            "ifixit/statsd-php-client": "dev-master"
            ...
    }
```

It would be great if you could add the package to Packagist.org
http://grossi.io/2013/creating-your-first-composer-packagist-package/