Added the psycopg event broker

[agronholm]

Changes

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #999, the entry should look like this:

* Fix big bad boo-boo in the async scheduler (#999 <https://github.com/agronholm/apscheduler/issues/999>_; PR by Yourname)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

If possible, use your real name in the changelog entry. If not, use your GitHub
username.

[coveralls]

coverage: 92.393% (-0.03%) from 92.426%
when pulling 87088d5 on psycopg-broker
into 60e9225 on master.