Add map type property (standard, satellite, hybrid) to MapView.

Summary:
Here is an example project demonstrating this pull request: [AKMapView](https://github.com/adamkrell/AKMapView)
Closes https://github.com/facebook/react-native/pull/1503
Github Author: Adam Krell <akrell@bethanyassociates.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Components/MapView/MapView.js

@@ -82,6 +82,19 @@ var MapView = React.createClass({
      */
     scrollEnabled: React.PropTypes.bool,
 
+    /**
+     * The map type to be displayed.
+     * 
+     * - standard: standard road map (default)
+     * - satellite: satellite view
+     * - hybrid: satellite view with roads and points of interest overlayed
+     */
+    mapType: React.PropTypes.oneOf([
+      'standard', 
+      'satellite', 
+      'hybrid',
+    ]),
+
     /**
      * The region to be displayed by the map.
      *