Switch all docstrings to sphinx conventions.
Support enumerating publications and namespaces.

The Manager can now enumerate the set of currently known publications
and namespaces.  Tests verify this.  If a client wants to monitor this
over time, the best it can do is poll with this API.
Add 'dist' to .gitignore.
Fix some pylint whitespace problems.
Include pygazebo.msg in packages and document them some.