add binary, fix #247

cmd.js is mostly taken from node_modules/electron-prebuilt/cli.js (what the node_modules/.bin/electron symlink points to)

package.json

@@ -19,6 +19,7 @@
     "debug": "^2.2.0",
     "drag-drop": "^2.11.0",
     "electron-localshortcut": "^0.6.0",
+    "electron-prebuilt": "0.37.2",
     "hyperx": "^2.0.2",
     "main-loop": "^3.2.0",
     "mkdirp": "^0.5.1",
@@ -34,7 +35,6 @@
   "devDependencies": {
     "electron-osx-sign": "^0.3.0",
     "electron-packager": "^5.0.0",
-    "electron-prebuilt": "0.37.2",
     "electron-winstaller": "^2.0.5",
     "gh-release": "^2.0.3",
     "plist": "^1.2.0",
@@ -64,5 +64,8 @@
     "start": "electron .",
     "test": "standard",
     "update-authors": "./bin/update-authors.sh"
+  },
+  "bin": {
+    "webtorrent-app": "./bin/cmd.js"
   }
 }