Add Polyline support to MapView

Summary: Per issue #1925, add support for Polyline to MapView.

Briefly, if you have a MapView declared as:

    <MapView
      annotations={this.state.annotations}
      overlays={this.state.overlays}
      style={styles.map}
      region={this.state.region}
      ref="mapView"
    />

then setting

    this.state.overlays = [{
      coordinates: [
        { latitude: 35.5, longitude: -5.5 },
        { latitude: 35.6, longitude: -5.6 },
        ...
      ],
      strokeColor: 'rgba(255, 0, 0, 0.5)',
      lineWidth: 3,
    }];

will draw a red line between the points in locations with a width of 3 and equally blended with the background.
Closes https://github.com/facebook/react-native/pull/4153

Reviewed By: svcscm

Differential Revision: D2697347

Pulled By: nicklockwood

fb-gh-sync-id: a436e4ed8d4e43f2872b39b4694fad7c02de8fe5

React/Views/RCTMapAnnotation.h

@@ -0,0 +1,21 @@
+/**
+ * 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.
+ */
+
+#import <MapKit/MapKit.h>
+
+@interface RCTMapAnnotation : MKPointAnnotation <MKAnnotation>
+
+@property (nonatomic, copy) NSString *identifier;
+@property (nonatomic, assign) BOOL hasLeftCallout;
+@property (nonatomic, assign) BOOL hasRightCallout;
+@property (nonatomic, assign) BOOL animateDrop;
+@property (nonatomic, strong) UIColor *tintColor;
+@property (nonatomic, strong) UIImage *image;
+
+@end