WebTorrent process

* Separate hidden window, with its own renderer process, for WebTorrent
  (Must be a window. You cannot run WebRTC at all in a Web Worker, and you can't
   run it well in a node process like the electron main process.)

* Disabled the create-torrent-modal for now. That gives us a consistent UX
  regardless of whether the user dragged files or folders onto the app or opened
  the Create New Torrent menu item.

* Main process routes all messages between the main and webtorrent windows.

* The renderer index.js is smaller now (but still too big), with the WebTorrent
  interface moved to webtorrent.js / it's own process.

* The UI should be faster now, and should not lag under load.

config.js

@@ -65,6 +65,7 @@ module.exports = {
   },
 
   WINDOW_ABOUT: 'file://' + path.join(__dirname, 'renderer', 'about.html'),
+  WINDOW_WEBTORRENT: 'file://' + path.join(__dirname, 'renderer', 'webtorrent.html'),
   WINDOW_MAIN: 'file://' + path.join(__dirname, 'renderer', 'main.html')
 }