Extend defaultStyle for activeStyle

There is a bug with the defaultStyle not getting all the style elements.
This is noticed when you are working with the `font` object inside activeStyle in combination with icons.

Example:
``` javascript
Styles.set('ah-no-style', {
	font: {
		fontWeight: 'normal',
		fontSize: '15dp'
	},
	activeStyle: {
		color: '#FF0000'
	}
});
```

Without this fix the icon activeStyle properties (https://github.com/FokkeZB/nl.fokkezb.button/blob/master/controllers/icon.js#L95) would be set to:
``` javascript
{ 
    color: '#FF0000', 
    font: {
        fontFamily: 'FontAwesome'
    }
}
```

But you want it to be:
``` javascript
{ 
    color: '#FF0000', 
    font: {
        fontWeight: 'normal',
        fontSize: '15dp',
        fontFamily: 'FontAwesome'
    }
}
```

Now, I'm sure there is a reason you are using _.pick so maybe this isn't the most efficient fix. But this works for us for now. Let me know how to do it more cleanly, if needed.