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.
bikeshed bikeshed: I often end up moving from `IsValid() bool` to `Validate() error`, and finding it more comfortable, but YMMV