WIP: Add discovery directly

Hello!

This is a work in progress that I will clean up tomorrow and submit as a real pull request but I wanted some initial feedback. Essentially I really like the discovery API on GAE but I want to have them available for my non-GAE projects. This will eventually allow go-endpoints to serve both the SPI backend on GAE and discovery directly.

```
http://localhost:8080/api/discovery/v1/apis/
http://localhost:8080/api/discovery/v1/api/discovery/v1/rest
http://localhost:8080/api/discovery/v1/api/account/v1/rest
http://localhost:8080/api/account/v1/groups
```

Thoughts?

Thanks.