Autogenerate topics.rst from fedmsg.meta test suite.

- Occurs during sphinx-build.
- Fixes #117.
- Requires https://github.com/fedora-infra/fedmsg_meta_fedora_infrastructure/pull/12
Require fedmsg.meta for rtfd to build the docs.
Comment tweak.
os.path.join
PEP8
Merge pull request #129 from fedora-infra/feature/topics-doc

Feature/topics doc