WIP: Port ranges provider support

Working on https://bugs.launchpad.net/juju-core/+bug/1337817 - enabling port range support in juju-core. This branch modifies the provider interface and its implementations to handle port ranges. They are silently converted into old-style port slices on the firewaller. Port range support in the firewaller is still in progress.
I'd just like to check: are we storing this struct in the database at all? If so we shouldn't include types defined outside the state package -- it's way too easy for someone to accidentally change our db schema by modifying distant code.