[Enhancement] Supply list of classes to v-class

Currently it's not possible to apply multiple classes in the `v-class` directive without specifying each class statically as keys. This pull request enables the parameter to `v-class` to be a space separated list of classes to apply, which can be built up within a computed property (or similar).

Example use case:

```js
new Vue({
    el: '#shell',
    
    data: function() {
        return {
            type: 'foo',
            subtype: 'bar'
        }
    },
    
    computed: {
        classList: function() {
            return [this.type, this.subtype].join(' ');
        }
    }
});
```

```html
<div id="shell" v-class="classList"></div>
```

Fiddle here: http://jsfiddle.net/o2twwvsn/