Configuration can now be loaded from ENV as well as from a configuration file

If you run stats.js without specifying a configuration file, configuration will be loaded from environment variables name-spaced with STATSD_:

```bash
STATSD_graphitePort=2003
STATSD_graphiteHost=carbon.hostedgraphite.com
STATSD_port=8125
STATSD_graphite_legacyNamespace=false
STATSD_graphite_globalPrefix=bananas
STATSD_flushInterval=5000
```

Creates

```javascript
{ graphitePort: 2003,
 graphiteHost: 'carbon.hostedgraphite.com',
port: 8125,
graphite: { 
legacyNamespace: false,
globalPrefix: 'bananas' 
},
flushInterval: 1000 
}
```

I tend to use foreman and a .env file to execute services, such as StatsD. This patch makes this option much more viable for me.