docutil: Add Transport.Has() for inclusion judgements

oas2: Add support for content negotiation

Now we can request an API document in JSON format. For example (via HTTPie):

```
$ http GET :8080/api Accept:application/json
$ http GET :8080/api?accept=json
```

generator: Improve the HTTP-server generator

Do not generate deprecated `NewHTTPRouterWithOAS` any more.

Update README

Update README

Merge pull request #21 from RussellLuo/minor-syntax-improvements

Minor syntax improvements

Improve the OAS package to handle recursive types properly

Tweaks

Update README

Cleanup httpapp