Custom annotation views for Android.

This gives feature parity to Custom Annotation Views
between iOS and Android.

- Add custom marker view and options for mapbox.
- Add an implementation for the Annotation class.
- Add a MarkerViewAdapater that transforms Annotations into views that
  get displayed on the map.
- Handle adding and removing of children.
- Update custom annotation docs.

android/src/main/java/com/mapbox/reactnativemapboxgl/RNMGLAnnotationViewManager.java

@@ -0,0 +1,56 @@
+package com.mapbox.reactnativemapboxgl;
+
+import android.util.Log;
+
+import com.facebook.react.bridge.ReadableMap;
+import com.facebook.react.uimanager.LayoutShadowNode;
+import com.facebook.react.uimanager.ThemedReactContext;
+import com.facebook.react.uimanager.ViewGroupManager;
+import com.facebook.react.uimanager.annotations.ReactProp;
+import com.mapbox.mapboxsdk.geometry.LatLng;
+
+import java.util.Map;
+
+/**
+ * Created by gcole on 2/17/17.
+ */
+public class RNMGLAnnotationViewManager extends ViewGroupManager<RNMGLAnnotationView> {
+    private LayoutShadowNode _shadowNode;
+
+    private static final String NAME = "RCTMapboxAnnotation";
+
+    @Override
+    public String getName() {
+        return NAME;
+    }
+
+    @Override
+    protected RNMGLAnnotationView createViewInstance(ThemedReactContext reactContext) {
+        return new RNMGLAnnotationView(reactContext, this);
+    }
+
+    @Override
+    public LayoutShadowNode createShadowNodeInstance() {
+        _shadowNode = super.createShadowNodeInstance();
+        return _shadowNode;
+    }
+
+    public LayoutShadowNode getShadowNode() {
+        return _shadowNode;
+    }
+
+    // Props
+
+    @ReactProp(name = "id")
+    public void setAnnotationId(RNMGLAnnotationView view, String value) {
+        view.setAnnotationId(value);
+    }
+
+    @ReactProp(name = "coordinate")
+    public void setCoordinate(RNMGLAnnotationView view, ReadableMap map) {
+        LatLng coordinate = new LatLng();
+        coordinate.setLatitude(map.getDouble("latitude"));
+        coordinate.setLongitude(map.getDouble("longitude"));
+        view.setCoordinate(coordinate);
+    }
+}