Declare namespace package.

This will stop pkg_resources from emitting unsettling warnings like this one:

  /usr/lib/python2.6/site-packages/fedmsg/meta/__init__.py:94: UserWarning:
        Module backports was already imported from
        /usr/lib64/python2.6/site-packages/backports/__init__.pyc, but
        /usr/lib/python2.6/site-packages is being added to sys.path