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

ReactAndroid/src/main/java/com/facebook/react/touch/ReactHitSlopView.java

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.touch;
+
+import android.graphics.Rect;
+
+import javax.annotation.Nullable;
+
+/**
+ * This interface should be implemented by all {@link View} subclasses that want to use the
+ * hitSlop prop to extend their touch areas.
+ */
+public interface ReactHitSlopView {
+
+  /**
+   * Called when determining the touch area of a view.
+   * @return A {@link Rect} representing how far to extend the touch area in each direction.
+   */
+  public @Nullable Rect getHitSlopRect();
+
+}