Make UUID an array type

```
This is more of a question than a bug, but why not make the UUID a `[16]byte` 
type, instead of the slice, `[]byte`?

Alternatively, to make it backwards-compatible, it could be a pointer type, 
`*[16]byte`.
```

Original issue reported on code.google.com by `attilaolah` on 16 Dec 2014 at 2:58
