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/)


Maybe call this `AssertStatN` to be consistent with the other names in the package and with Go stdlib patterns [e.g., SplitN](http://golang.org/src/pkg/bytes/bytes.go?s=4912:4954#L223)