Checkboxes with images

This should fix #178 that was affected by #192.
Also, I found out that comparing html value is not reliable, even ignoring the case (as f0d4fca2f3bfadd1297c17b7c5e53ea55b69af30).

For example, in Chrome the original html is: 
```html
<img alt="No" src="/assets/no.png" />
```
But what is returned by `this.element.html()` is
```html
<img alt="No" src="/assets/no.png">
```

So, I changed the way checkboxes handle their value. Now it works like selects, using collectionValue.