Supporting _all_docs in the SDK

The Python SDK currently doesn't seem to support _all_docs at the moment.

I remember that it was possible in the past with something like `.view("_all_docs")`, but I think this is the wrong way to do it. _all_docs should really have its own method. The reasons are:

• _all_docs and Views are a different thing, this should not be hidden to the user
• _all_docs uses raw collation and not Unicode collation as Views do
• I'm not sure about this one, but it could be the case, that _all_docs also contains not yet persisted items (as opposed to views).