Tor Hidden Service

These changes are on top of #15 (note: I rebased to the current master branch).

Add the following subcommands to the tor action:
* get-hs: Get a string with configured hidden service hostname and ports.
* enable-hs: If no hidden service is configured, enable one for HTTP/HTTPS.
* disable-hs: If a hidden service was previously configured by Plinth, then disable it.

Also rearranged the Tor module information and added a simple form to enable/disable the hidden service.