events: Support prototype property name as an event type

The prototype property name (such as '__proto__') as an event type does not work as my expectation.

```
> var events = require('events')
> var e = new events.EventEmitter();
> e.on('__proto__', function() {});
> e.emit('__proto__');
TypeError: Object #<Object> has no method 'apply'
    at EventEmitter.emit (events.js:126:20)
    at repl:1:3
    at REPLServer.self.eval (repl.js:109:21)
    at rli.on.self.bufferedCmd (repl.js:258:20)
    at REPLServer.self.eval (repl.js:116:5)
    at Interface.<anonymous> (repl.js:248:12)
    at Interface.EventEmitter.emit (events.js:96:17)
    at Interface._onLine (readline.js:200:10)
    at Interface._line (readline.js:518:8)
    at Interface._ttyWrite (readline.js:736:14)
```

To support these, use `Object.create(null)` instead of `{}`.