Thanks to @richardsheridan for pointing out the limitations of using
*any* kind of value as the result-cached-flag and how it might cause
problems for anyone returning pickled blob-data. This changes the
`Portal` internal result value tracking to stash the full message from
which the value can be retrieved by any `Portal.result()` caller.
The internal change is that `Portal._return_once()` now returns a tuple
of the message *and* its value.
Add a small config with a manually specified version key for now.
Fix up some changelog contents from last release and bump our `setup.py`
version to an `.alpha2.dev0`.
Resolves#227