Provide access to _changes endpoint

[ryanjyoder]

API should provide access to the _changes endpoint.
http://docs.couchdb.org/en/2.0.0/api/database/changes.html

Here's my first crack at it. Thoughts?
https://github.com/ryanjyoder/couchdb/tree/add_changes_endpoint
https://github.com/ryanjyoder/couchdb/blob/add_changes_endpoint/changes.go (most change here)
Alternatively this logic could be added directly to Database?

[zemirco]

good point 👍

I once wrote the follow package but nowadays I think it doesn't make sense to have this features in a separate package. Will move the feature into this couchdb package.