Use f.button :submit on simple_form templates to get the btn class.

Adds a 'btn' css class to the submit input if one is using Simple Form.

E.g. turns:

```html
<input name="commit" type="submit" value="Set my password">
```

to this:

```html
<input class="btn" name="commit" type="submit" value="Set my password">
```

It makes a nice bootstrap styled button. Devise also does this in it's simple form for generators.