API discovery: add a REST API to list the API

[FredericHeem]

HATEOAS, Google API Discovery Service or another ?

[tomsiwik]

Mind tagging / versioning or developer/master-branch this repo? I'm scared you put something awesome inside that breaks my own project and/or i can't include new features.

[claudio-viola]

Reference: https://developer.github.com/v3/

[tomsiwik]

👍 for the tags. Thanks alot.