UniterAPI: OpenPorts and ClosePorts added

Introduced OpenPorts and ClosePorts uniter API
methods, which allow opening and closing port
ranges on units. The existing OpenPort and ClosePort
methods are left for backwards-compatibility, but
internally they use the new methods.

NOTE: Already approved as http://reviews.vapour.ws/r/111/
This PR just re-targets the branch against juju:master