Save custom rooms' keys so the user can re-broadcast to it without setting a custom password

[vitorbaptista]

By default, unless you specify a password, shellshare generates a random one every time it starts broadcasting. This is fine for random room names, but can be confusing for custom rooms (see #36). I want to be able do to:

$ python shellshare --room vitorbaptista
Sharing terminal in https://shellshare.net/r/vitorbaptista
$ # ...
$ exit
End of transmission
$ # ...
$ python shellshare --room vitorbaptista
Sharing terminal in https://shellshare.net/r/vitorbaptista

To do so, we must either save this random password, or use a unique identifier for the user's machine. I would prefer to avoid dealing with saving/loading files, so something like a MAC address should work fine. Maybe there're some people that would be uncomfortable in sharing this information with a third-party, but considering you're sharing your whole terminal, it should be OK.

[vitorbaptista]

uuid.getnode() seems like a great candidate for a password.