Add option to return metadata with the raw message

With this change one can specify one or several `meta` arguments to
the request. The corresponding metadata will be added to the
raw_message under the key `meta`.
Sets take a list.
Indentation.
Return the argued meta attributes back to the user.
Fix checking that the meta provided are part of the allowed set

The intersection between the list of meta and the allowed meta should
be equal to the length of the list of meta provided. Otherwise, at least
one of the meta provided is not in the list of allowed meta.
Usernames should be plural here.
Initialize fedmsg metadata processors at startup.
fedmsg.meta is expecting a dict.
Let flask handle listification for us.
Convert messages from sqlalchemy objects to json-like dicts earlier in the pipeline so we can manipulate them.
Pluralization.
Fix up the runserver script.
Convert set to list before trying to serialize.

https://github.com/fedora-infra/fedmsg/pull/139