io.Writer wrapper for statsd-secure

```go
conn, err := net.DialTimeout("udp", "endpoint.example.com:8126", 2*time.Second)
if err != nil {
  // handle err
}

writer := &opstocat.StatsdSignedWriter{Writer: conn, Key: []byte("supersecret")}
statter := g2s.New(writer)
statter.Counter(1.0, "foo", 1) // :banana: out
```

For review: @technoweenie @vmg