Support binding to top level keys

So I noticed this right away and wanted to fix it:

HTML
```html
<div id="broken">
  <input type="text" data-value="name"/>
</div>

<div id="works">
  <input type="text" data-value=".name"/>
</div>

<div id="also-works">
  <input type="text" data-value="model.name"/>
</div>
```

Coffee/jQ
```coffee
model = {name: '@dashkb'}
rivets.bind ($ '#broken'), model
rivets.bind ($ '#works'), model
rivets.bind ($ '#also-works'), model: model
```

I am not sure how opinionated you want Rivets to be but seems to me all of the above cases should work.

Also the spec might belong someplace else; but I wanted to stick with your "do everything in the before hook" style.