Add scrollToEnd to ScrollView and ListView

Summary:
**Motivation**

A basic task of making a React Native ScrollView or ListView scroll to the bottom is currently very hard to accomplish:
- https://github.com/facebook/react-native/issues/8003
- https://github.com/facebook/react-native/issues/913
- http://stackoverflow.com/questions/29829375/how-to-scroll-to-bottom-in-react-native-listview

**NOTE:** If you're building something like a chat app where you want a ListView to keep scrolling to the bottom at all times, it's easiest to use the [react-native-invertible-scrollview](https://github.com/exponent/react-native-invertible-scroll-view) component rather calling `scrollToEnd` manually when layout changes. The invertible-scrollview uses a clever trick to invert the direction of the ScrollView.

This pull request adds a `scrollToEnd` method which scrolls to the bottom if the ScrollView is vertical, to the right if the ScrollView is horizontal.

The implementation is based on this SO answer:
http://stackoverflow.com/questions/952412/uiscrollview-scrol
Closes https://github.com/facebook/react-native/pull/12088

Differential Revision: D4474974

Pulled By: mkonicek

fbshipit-source-id: 6ecf8b3435f47dd3a31e2fd5be6859062711c233

Examples/UIExplorer/js/ScrollViewExample.js

@@ -57,6 +57,11 @@ exports.examples = [
           onPress={() => { _scrollView.scrollTo({y: 0}); }}>
           <Text>Scroll to top</Text>
         </TouchableOpacity>
+        <TouchableOpacity
+          style={styles.button}
+          onPress={() => { _scrollView.scrollToEnd({animated: true}); }}>
+          <Text>Scroll to bottom</Text>
+        </TouchableOpacity>
       </View>
     );
   }
@@ -79,6 +84,11 @@ exports.examples = [
           onPress={() => { _scrollView.scrollTo({x: 0}); }}>
           <Text>Scroll to start</Text>
         </TouchableOpacity>
+        <TouchableOpacity
+          style={styles.button}
+          onPress={() => { _scrollView.scrollToEnd({animated: true}); }}>
+          <Text>Scroll to end</Text>
+        </TouchableOpacity>
       </View>
     );
   }