Add hitSlop prop on iOS and Android

Summary:New prop `hitSlop` allows extending the touch area of Touchable components. This makes it easier to touch small buttons without needing to change your styles.

It takes `top`, `bottom`, `left`, and `right` same as the `pressRetentionOffset` prop. When a touch is moved, `hitSlop` is combined with `pressRetentionOffset` to determine how far the touch can move off the button before deactivating the button.

On Android I had to add a new file `ids.xml` to generate a unique ID to use for the tag where I store the `hitSlop` state. The iOS side is more straightforward.

terribleben worked on the iOS and JS parts of this diff.

Fixes #110
Closes https://github.com/facebook/react-native/pull/5720

Differential Revision: D2941671

Pulled By: androidtrunkagent

fb-gh-sync-id: 07e3eb8b6a36eebf76968fdaac3c6ac335603194
shipit-source-id: 07e3eb8b6a36eebf76968fdaac3c6ac335603194

Libraries/Components/View/View.js

@@ -11,6 +11,7 @@
  */
 'use strict';
 
+const EdgeInsetsPropType = require('EdgeInsetsPropType');
 const NativeMethodsMixin = require('NativeMethodsMixin');
 const PropTypes = require('ReactPropTypes');
 const React = require('React');
@@ -201,6 +202,19 @@ const View = React.createClass({
     onMoveShouldSetResponder: PropTypes.func,
     onMoveShouldSetResponderCapture: PropTypes.func,
 
+    /**
+     * This defines how far a touch event can start away from the view.
+     * Typical interface guidelines recommend touch targets that are at least
+     * 30 - 40 points/density-independent pixels. If a Touchable view has a
+     * height of 20 the touchable height can be extended to 40 with
+     * `hitSlop={{top: 10, bottom: 10, left: 0, right: 0}}`
+     * ** NOTE **
+     * The touch area never extends past the parent view bounds and the Z-index
+     * of sibling views always takes precedence if a touch hits two overlapping
+     * views.
+     */
+    hitSlop: EdgeInsetsPropType,
+
     /**
      * Invoked on mount and layout changes with
      *