Added an autoload option

Firstly - awesome library! Thank you for all your hard work!

This pull request adds an option to both the global Howler and Howl objects allowing a user to defer loading of a sound if they want to. Usage examples:

```js
// By default autoload is set to true globally as to not break existing implementations
Howler.autoload = false;

// User must now load each Howl individually, or all at once using the global object
Howler.load()

for (var i = 0; i < howls.length; i++) {
    howls[i].load();
}

```

Optionally, a user can specify a single Howl to not autoload:

```js
var howl =  new Howl({
    autoload: false,
    urls: ['sound.mp3']
});

//... Do some other processing here

howl.load()
```