Vidalia should let Torbutton NEWNYM

I think we're getting closer to a working vision for implementing NEWNYM support in the browser in ticket #523 (closed). We basically have two options:

1. Have vidalia pass Firefox the Control Port password in an ENV variable, so Torbutton could speak to the Tor Control Port to NEWNYM.

2. Create a 'Least Privilege' Control Port that does not require authentication, that implements a safe subset of the Tor control port, including SIGNAL NEWNYM.

Option 2 can be done either in Tor (hard), or as a proxy filter in Vidalia (easier).

Either of these options requires just about the same amount of work in Torbutton, and if done correctly, Torbutton should conceivably be able to use either Control Port, because they should both have the same exact syntax for the relevant commands and events.