Call library methods only when ready

Defers actual library method calls until driver is loaded and ready. This renders the `ready()` calls unnecessary when using with RequireJS.

```javascript
define(['localforage'], function(localforage) {
    // As a callback:
    localforage.ready(function() {
        localforage.setItem('mykey', 'myvalue', console.log);
    });

    // With a Promise:
    localforage.ready().then(function() {
        localforage.setItem('mykey', 'myvalue', console.log);
    });
});
```

The above RequireJS example from the readme could simply be the following in both cases:

```javascript
define(['localforage'], function(localforage) {
    localforage.setItem('mykey', 'myvalue', console.log);
});
```
It might be handy to add in some comments explaining what this does. I understand it and it's clever, but a bit opaque.

Also, is there a way we can automate this? It seems brittle to rely on adding methods in here and to each driver. I'm okay with it, but if we could do it with metaprogramming that'd be great.