Remove usage of Martini

Martini doesn't add a lot to this repository - given we're only using 3
endpoints it's simpler to just use plain Go net/http handlers.

- rewrite main.go to use plain http.ServeMux
- rewrite handlers.go to use http.ResponseWriter
- remove the Martini dependency
Port and validate previous routing behaviour

Ensure that HTTP method enforcement is preserved after removing Martini.
Merge pull request #12 from alphagov/remove-martini

Remove Martini framework