[Touchable] Change default activeOpacity to 0.2 to match iOS

Summary:
Closes https://github.com/facebook/react-native/pull/296
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Components/Touchable/TouchableOpacity.js

@@ -59,7 +59,7 @@ var TouchableOpacity = React.createClass({
 
   getDefaultProps: function() {
     return {
-      activeOpacity: 0.5,
+      activeOpacity: 0.2,
     };
   },