[WiP] Return status codes and headers

Plumbs the way toward returning additional headers (and non-200 status codes)
from deeper within.

- [x] implement status code → status + reason lookup
- [x] test CGI version
- [ ] test mod_python version
- [x] move header assembly into `Layer.getTile()`
- [x] fix `makeHeaders()`
- [x] switch from dicts to lists of tuples with helper methods