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.

main/index.js

@@ -54,6 +54,7 @@ function init () {
   app.on('ready', function () {
     menu.init()
     windows.createMainWindow()
+    windows.createWebTorrentHiddenWindow()
     shortcuts.init()
     tray.init()
     handlers.install()