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 feel like this chunk of logic could be quite nicely expressed as `network.CollapsePortRanges()` or something -- and it's easier to test when it's nicely exposed.