WIP live mode
unify the return values for all types of nodes

I decided it would be best to standardize on wrapping all return values
in a tuple. That way, code built around this can always do exactly one
thing (iterate over a result tuple.)

closes #3
partial fix for #2
add emit_immediatly to node options (closes #2)
cover live mode branch with a test
add message about emit_immediately to changelog
add documentation in API
Merge branch 'feature/live' into develop