[Idea: ScrollView] Add `getScrollResponder` to ScrollView for composition

Summary:
This is a proposal to add `getScrollResponder` to all ScrollView-like components, including ListView. This allows multiple higher-order scroll views to be composed while allowing the owner of the top-level scroll view to call `scrollableView.getScrollResponder().scrollTo(...)` regardless of whether `scrollableView` is a ScrollView, ListView, InvertedScrollView, etc.
Closes https://github.com/facebook/react-native/pull/766
Github Author: James Ide <ide@jameside.com>

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

Libraries/Components/ScrollView/ScrollView.js

@@ -200,6 +200,16 @@ var ScrollView = React.createClass({
     this.refs[SCROLLVIEW].setNativeProps(props);
   },
 
+  /**
+   * Returns a reference to the underlying scroll responder, which supports
+   * operations like `scrollTo`. All ScrollView-like components should
+   * implement this method so that they can be composed while providing access
+   * to the underlying scroll responder's methods.
+   */
+  getScrollResponder: function(): ReactComponent {
+    return this;
+  },
+
   getInnerViewNode: function(): any {
     return React.findNodeHandle(this.refs[INNERVIEW]);
   },