[React] Add support for arrays with holes in style attribute

**Summary:**
See the style proposal on react-future: https://github.com/reactjs/react-future/blob/master/04%20-%20Layout/Inline%20Style%20Extension.md

The goal is to enable writing CSS purely in JavaScript. We've been using this for a couple of internal projects and it's proven to be a very expressive tool.

Note that this diff doesn't bring `StyleSheet.create` nor `StylePropType`. I'm keeping it simple by just allowing arbitrary arrays, objects and falsy values.

Example of use:

```javascript
...
  render: function() {
...
    <div style={[styles.base, this.state.active && styles.active]} />
...

var styles = {
  base: { color: 'red' },
  active: { fontWeight: 'bold' }
};
```

**Test Plan:**

```
grunt test
```

**Reviewers:** jwalke zpao sema
**CC:** tomocchino