Definable distance pagination for ScrollView

Summary: This is an enhancement for ScrollView that adds the ability to paginate based on a width other than the width of the ScrollView itself. This is a fairly common pattern used on apps like Facebook, App Store, and Twitter to scroll through a horizontal set of cards or icons:

![img_8726 2](https://cloud.githubusercontent.com/assets/451050/8017899/39f9f47c-0bd2-11e5-9c1d-889452f20cf7.PNG) ![img_8727 2](https://cloud.githubusercontent.com/assets/451050/8017898/39f962dc-0bd2-11e5-98b4-461ac0f7f21b.PNG)  ![img_8728 2](https://cloud.githubusercontent.com/assets/451050/8017900/39fd91a4-0bd2-11e5-8786-4cf0316295a0.PNG)

After trying to accomplish this only with JS, it appears that attempting to take over an in-progress native scroll animation with JS is always going to result in some amount of jankiness and jumping.

This pull request uses `scrollViewWillEndDragging` in RCTScrollView.m to adjust `targetContentOffset` based on two new optional props added to ScrollView. `snapToInterval` sets the multiple that the
Closes https://github.com/facebook/react-native/pull/1532

Reviewed By: @​svcscm, @​trunkagent

Differential Revision: D2443701

Pulled By: @vjeux

Libraries/Components/ScrollView/ScrollView.js

@@ -246,6 +246,27 @@ var ScrollView = React.createClass({
      */
     stickyHeaderIndices: PropTypes.arrayOf(PropTypes.number),
     style: StyleSheetPropType(ViewStylePropTypes),
+    /**
+     * When set, causes the scroll view to stop at multiples of the value of
+     * `snapToInterval`. This can be used for paginating through children
+     * that have lengths smaller than the scroll view. Used in combination
+     * with `snapToAlignment`.
+     * @platform ios
+     */
+    snapToInterval: PropTypes.number,
+    /**
+     * When `snapToInterval` is set, `snapToAlignment` will define the relationship
+     * of the the snapping to the scroll view.
+     *   - `start` (the default) will align the snap at the left (horizontal) or top (vertical)
+     *   - `center` will align the snap in the center
+     *   - `end` will align the snap at the right (horizontal) or bottom (vertical)
+     * @platform ios
+     */
+    snapToAlignment: PropTypes.oneOf([
+      'start', // default
+      'center',
+      'end',
+    ]),
     /**
      * Experimental: When true, offscreen child views (whose `overflow` value is
      * `hidden`) are removed from their native backing superview when offscreen.
@@ -430,6 +451,8 @@ var validAttributes = {
   scrollsToTop: true,
   showsHorizontalScrollIndicator: true,
   showsVerticalScrollIndicator: true,
+  snapToInterval: true,
+  snapToAlignment: true,
   stickyHeaderIndices: {diff: deepDiffer},
   scrollEventThrottle: true,
   zoomScale: true,