Allow overriding low level message on a per request basis
Update doc to reflect latest changes on low level messages
Update execute and stream signatures to return result first, then response

This is a backward incompatible change.

This commit also removes the Stream "result" event and updates the
signature of the "data" event to (result, response) instead of
(response).
Add client options for op, processor and accept
Update docs and changelog
Bump to v0.2.0