Add a `WildEmitter.mixin(<constructor>)` method for mixing emitter behaviour into a "class" more easily.

So I love WildEmitter, but I can literally never remember how to actually use it. And even when I read the docs I forget that I have to both inherit from a new Emitter, and do Emitter.call(this) in my constructor, I always forget one or the other.

This PR makes it as easy as

```javascript
var Emitter = require('wildemitter');

function Fruit () {
  //My own constructor code
}

Emitter.mixin(Fruit)
```

And doesn't break backwards compatibility.