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.
      *

React/Views/RCTConvert+MapKit.h

@@ -15,6 +15,7 @@
 + (MKCoordinateSpan)MKCoordinateSpan:(id)json;
 + (MKCoordinateRegion)MKCoordinateRegion:(id)json;
 + (MKShape *)MKShape:(id)json;
++ (MKMapType)MKMapType:(id)json;
 
 typedef NSArray MKShapeArray;
 + (MKShapeArray *)MKShapeArray:(id)json;

React/Views/RCTConvert+MapKit.m

@@ -43,4 +43,10 @@
 
 RCT_ARRAY_CONVERTER(MKShape)
 
+RCT_ENUM_CONVERTER(MKMapType, (@{
+  @"standard": @(MKMapTypeStandard),
+  @"satellite": @(MKMapTypeSatellite),
+  @"hybrid": @(MKMapTypeHybrid),
+}), MKMapTypeStandard, integerValue)
+
 @end

React/Views/RCTMapManager.m

@@ -41,6 +41,7 @@ RCT_EXPORT_VIEW_PROPERTY(scrollEnabled, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(maxDelta, CGFloat)
 RCT_EXPORT_VIEW_PROPERTY(minDelta, CGFloat)
 RCT_EXPORT_VIEW_PROPERTY(legalLabelInsets, UIEdgeInsets)
+RCT_EXPORT_VIEW_PROPERTY(mapType, MKMapType)
 RCT_EXPORT_VIEW_PROPERTY(annotations, MKShapeArray)
 RCT_CUSTOM_VIEW_PROPERTY(region, MKCoordinateRegion, RCTMap)
 {