export: Default user name to app name

and remove code for deducing current user.

This makes the behavior consistent with Foreman (see #109). Also, I
think folks are more likely to want to run their apps as some
app-specific user, not as the user running export. In fact, some folks
may run `honcho export` on a completely different box than where they
end up running the applications.
Merge pull request #111 from msabramo/export_default_user_to_app

export: Default user name to app name