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)

bin/cmd.js

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+
+var electron = require('electron-prebuilt')
+var proc = require('child_process')
+var path = require('path')
+
+var child = proc.spawn(electron, [path.join(__dirname, '..')], {stdio: 'inherit'})
+child.on('close', function (code) {
+  process.exit(code)
+})