[Idea] Add an API version header to all responses

This small change adds an `X-Zinc-API-Version` header to all responses. The reasoning is that it would allow end users to specify a catalog reference like

```
zinc+http://127.0.0.1:5000/com.mindsnacks.games
```

The zinc client would interpret the 'zinc+http' portion as a zinc REST API. The client could then do a HEAD request and determine the exact API version charge onward.