Add package-centric caching routines to fmn.rules.utils.

    >>> import fedmsg.config
    >>> import fmn.rules.utils
    >>> config = fedmsg.config.load_config()
    >>> fmn.rules.utils.get_packagers_of_package(config, 'nethack')
    set([u'lmacken', u'group::provenpackager'])
Use package-centric caching routines.
Safety first.
Merge pull request #10 from fedora-infra/feature/package-centric-caching

Use package-centric caching routines.