[WIP] 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.

WIP: For early review and approval; This cannot land
before its parent dimitern:#2 is merged into juju:master.