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/Touchable/TouchableHighlight.js

@@ -176,6 +176,10 @@ var TouchableHighlight = React.createClass({
     return this.props.pressRetentionOffset || PRESS_RETENTION_OFFSET;
   },
 
+  touchableGetHitSlop: function() {
+    return this.props.hitSlop;
+  },
+
   touchableGetHighlightDelayMS: function() {
     return this.props.delayPressIn;
   },
@@ -230,6 +234,7 @@ var TouchableHighlight = React.createClass({
         ref={UNDERLAY_REF}
         style={this.state.underlayStyle}
         onLayout={this.props.onLayout}
+        hitSlop={this.props.hitSlop}
         onStartShouldSetResponder={this.touchableHandleStartShouldSetResponder}
         onResponderTerminationRequest={this.touchableHandleResponderTerminationRequest}
         onResponderGrant={this.touchableHandleResponderGrant}