use Subprocess.jsm to launch tor

In Firefox 49, Mozilla added a new Subprocess.jsm module (they use it to implement the WebExtensions Native Messaging API). Kathy and I have not looked at it closely yet, but this module may provide a better way for Tor Launcher to start and manage the tor process. In particular, Subprocess.jsm provides access to stderr which would allow Tor Launcher to capture all output produced by tor.

Docs: https://firefox-source-docs.mozilla.org/toolkit/modules/subprocess/toolkit_modules/subprocess/index.html