(Reintroduce) API documentation

[rufuspollock]

As part of refactor for CKAN v2.9 #6 and resource versioning #7 we removed API docs 4304115

We want to add that back (and update it) once API is stabilized.

[tomeksabala]

@rufuspollock Are there any rough estimates on when a CKAN 2.9 & activity stream friendly version of this extension will be done?
As you probably know my team is trying to achieve a very similar goal. We could help or at least try to plan our schedule to integrate ckanext-versions into it.