feat(buttons): added checkbox and radio buttons

I have created directives for checkbox and radio buttons respectively. I have a few things to discuss:

1) Should I create a directive which can toggle on click a custom name? (Semantic-UI has an example as the first button. When a button has a name once its cliked name changes) - as I guess I will need to use transclude
2) is the naming style of directives good?
3) Not so sure about docs style. I saw yours as a guide. Is that alright?

Hope it would be fine!
It means that you can have markup like this:

```js
<form ng-model-options="{ some...stuff }">
  <input ...>
  <input ...>
</form>
```

and all of the inputs would use the ngModelOptions from the form, unless they were overridden with their own. It's very clever.