Text highlighting on iOS

Summary:
This diff implements highlighting of tapped text subranges for the iOS `<Text>` component, styled to match how iOS webkit views highlight links (translucent grey overlay with rounded corners).

Highlighting is enabled by default for any `<Text>` component which has an onPress handler. To disable the highlight, add `suppressHighlighting={true}` to the component props.

Examples/UIExplorer/TextExample.ios.js

@@ -269,7 +269,7 @@ exports.examples = [
           right right right right right right right right right right right right right
         </Text>
         <Text style={{textAlign: 'justify'}}>
-          justify: this text component's contents are laid out with "textAlign: justify"
+          justify: this text component{"'"}s contents are laid out with "textAlign: justify"
           and as you can see all of the lines except the last one span the
           available width of the parent container.
         </Text>
@@ -369,6 +369,15 @@ exports.examples = [
       </View>
     );
   },
+}, {
+  title: 'Text highlighting (tap the link to see highlight)',
+  render: function() {
+    return (
+      <View>
+        <Text>Lorem ipsum dolor sit amet, <Text suppressHighlighting={false} style={{backgroundColor: 'white', textDecorationLine: 'underline', color: 'blue'}} onPress={() => null}>consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud</Text> exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</Text>
+      </View>
+    );
+  },
 }];
 
 var styles = StyleSheet.create({