Use more efficient packed encoding for repeated numeric fields

See https://developers.google.com/protocol-buffers/docs/proto#simple under "Specifying Field Rules"

```
For historical reasons, repeated fields of basic numeric types aren't encoded as efficiently as they could be. New code should use the special option [packed=true] to get a more efficient encoding.
```
Merge pull request #13 from jplock/jp-packed-encoding

Use more efficient packed encoding for repeated numeric fields