Export `Animated.Node` for ease of type checking

Summary:
At present, there's no uniform way of determining whether you've received an Animated value node which can be attached to an animated prop.

You can attempt to use `instanceof Animated.Value`, but this fails for interpolated values. You can use `instanceof Animated.Interpolation`, but this fails for the values that are returned from the math functions (e.g., `Animated.Add`).

This commit exposes the base type from which all of these value nodes derive. As such, it is now possible to do the following:

```js
import React, { Component, PropTypes } from 'react';
import { Animated } from 'react-native';

class Widget extends Component {
  static propTypes = {
    progress: PropTypes.instanceOf(Animated.Node).isRequired,
  }
  // ...
}
```

Unnecessary for cosmetic change.
Closes https://github.com/facebook/react-native/pull/14688

Differential Revision: D5581539

Pulled By: shergin

fbshipit-source-id: 98f40e63a463241c7f91c72391c26c3b4153e4cd

Libraries/Animated/src/AnimatedImplementation.js

@@ -2734,6 +2734,10 @@ module.exports = {
    * See also [`AnimatedInterpolation`](docs/animated.html#animatedinterpolation).
    */
   Interpolation: AnimatedInterpolation,
+  /**
+   * Exported for ease of type checking. All animated values derive from this class.
+   */
+  Node: Animated,
 
   /**
    * Animates a value from an initial velocity to zero based on a decay