use map to track stat commands


- [x] Version bumped (see semver.org)
- [x] CHANGELOG updated
- [ ] README updated (optional)
- [ ] Code reviewed
- [x] [Jenkins build passes](http://build.sendgrid.net:8080/job/go-statsdclient-pullrequests/12/)


Consider making `Commands` private if it's not needed to be exported.