feat(AgmMap): know when map is ready

You can now listen to the new mapReady output
to know when the map is initialized.

Closes #740

src/core/directives/map.ts

@@ -361,9 +361,9 @@ export class AgmMap implements OnChanges, OnInit, OnDestroy {
       mapTypeId: this.mapTypeId,
       clickableIcons: this.clickableIcons,
       gestureHandling: this.gestureHandling
-    }).then(() => {
-      this.mapReady.emit(this._mapsWrapper.getNativeMap());
-    });
+    })
+      .then(() => this._mapsWrapper.getNativeMap())
+      .then(map => this.mapReady.emit(map));
 
     // register event listeners
     this._handleMapCenterChange();