events: have events module exports EventEmitter

This change is 100% backwards compatible.

This change will make using `EventEmitter` slightly simpler / nicer and
adheres to the best practice set forth by substack.

```js
var EventEmitter = require("events")

var emitter = new EventEmitter()
```

The only difference is that we now have to set `EventEmitter` as a
property of `EventEmitter` for backwards compatibility like we do with
[`Stream`][1]

We have also set the `usingDomains` property on the `EventEmitter`
constructor itself because that aligns with it's current usage of
`require("events").usingDomains = true`

There are other internals that would benefit from this change as well
like `StringDecoder`