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.

package.json

@@ -17,8 +17,10 @@
     "airplay-js": "guerrerocarlos/node-airplay-js",
     "application-config": "^0.2.0",
     "application-config-path": "^0.1.0",
+    "bitfield": "^1.0.2",
     "chromecasts": "^1.8.0",
     "create-torrent": "^3.22.1",
+    "deep-equal": "^1.0.1",
     "dlnacasts": "^0.0.2",
     "drag-drop": "^2.11.0",
     "electron-localshortcut": "^0.6.0",