Make OperationalImporterPscSync#{import, reset} require a whodunnit.

OperationalImporterPscSync is used in different responsibility contexts,
so there's no appropriate default.  (In the merge code, it's the user
who submitted the candidate data; in the import tasks, it's run as a
general "import" user.)  Under this assumption, there's two
possibilities:

1) Keep the arity of #import and #reset, but raise an error if a
   responsible user is not set.
2) Change the arity of #import and #reset, which raises responsibility
   assignment to a syntax-level requirement.  (Sort of.  A check for a
   non-nil responsible user might also be useful.)

I chose #2.
Update merge and import for new OperationalImporterPscSync interface.
Strip trailing spaces.
Merge pull request #89 from NUBIC/require-pscsync-attribution

Require attribution in OperationalImporterPscSync#{import, reset}