Map follow user location

Summary:
Fix #3105

It's the same PR as #3119 but as I force-pushed in my branch, I can't reopen the PR. I added an example.
![capture d ecran 2016-01-05 a 07 15 37](https://cloud.githubusercontent.com/assets/1107936/12108841/2727f504-b37c-11e5-8250-b53785930aba.png)
Closes https://github.com/facebook/react-native/pull/5126

Reviewed By: svcscm

Differential Revision: D2803052

Pulled By: nicklockwood

fb-gh-sync-id: 2e8978ff1b293d699462a8290b45fa74cc16b4dd

Examples/UIExplorer/MapViewExample.js

@@ -290,9 +290,15 @@ exports.examples = [
     }
   },
   {
-    title: 'Map shows user location',
+    title: 'showsUserLocation + followUserLocation',
     render() {
-      return <MapView style={styles.map} showsUserLocation={true} />;
+      return (
+        <MapView
+          style={styles.map}
+          showsUserLocation={true}
+          followUserLocation={true}
+        />
+      );
     }
   },
   {