Add a development helper

This passes events into a service instance just as the Gerrit
listener does, except that they come from JSON files specified
on the command line instead of a stream.

Change-Id: I670c8149ae9f4b0c89bf560fd2530b8c2843a78f
Fix what is passed to the GroupMailer

I forgot to update its construction when I changed it to expecting
callables rather than objects.

Change-Id: Ie9af63bbbee9092d806bc776b8d1357a2f3543b1