added showOverlay temp API

src/Navigation.js

   return platformSpecific.startSingleScreenApp(params);
 }
 
+function showOverlay(params) {
+  return platformSpecific.showOverlay(params);
+}
+
 export default {
   getRegisteredScreen,
   registerComponent,
   showInAppNotification: showInAppNotification,
   dismissInAppNotification: dismissInAppNotification,
   startTabBasedApp: startTabBasedApp,
-  startSingleScreenApp: startSingleScreenApp
+  startSingleScreenApp: startSingleScreenApp,
+  showOverlay
 };

src/deprecated/platformSpecificDeprecated.android.js

   if (fab === null || fab === undefined) {
     return;
   }
-  if (Object.keys(fab).length === 0 ) {
+  if (Object.keys(fab).length === 0) {
     return {};
   }
 
   newPlatformSpecific.dismissContextualMenu();
 }
 
+function showOverlay(params) {
+  newPlatformSpecific.showOverlay(params);
+}
+
 export default {
   startTabBasedApp,
   startSingleScreenApp,
   navigatorToggleNavBar,
   showSnackbar,
   showContextualMenu,
-  dismissContextualMenu
+  dismissContextualMenu,
+  showOverlay
 };

src/deprecated/platformSpecificDeprecated.ios.js

                     titleImage={tab.titleImage}
                     component={tab.screen}
                     passProps={{
-                    navigatorID: tab.navigationParams.navigatorID,
-                    screenInstanceID: tab.navigationParams.screenInstanceID,
-                    navigatorEventID: tab.navigationParams.navigatorEventID
-                  }}
+                      navigatorID: tab.navigationParams.navigatorID,
+                      screenInstanceID: tab.navigationParams.screenInstanceID,
+                      navigatorEventID: tab.navigationParams.navigatorEventID
+                    }}
                     style={tab.navigationParams.navigatorStyle}
                     leftButtons={tab.navigationParams.navigatorButtons.leftButtons}
                     rightButtons={tab.navigationParams.navigatorButtons.rightButtons}
     navigatorEventID,
     navigatorID
   };
-  
+
   savePassProps(params);
 
   Notification.show({
   // Android only
 }
 
+function showOverlay(params) {
+  showInAppNotification(params);
+}
+
 export default {
   startTabBasedApp,
   startSingleScreenApp,
   navigatorSwitchToTab,
   navigatorToggleNavBar,
   showContextualMenu,
-  dismissContextualMenu
+  dismissContextualMenu,
+  showOverlay
 };

src/platformSpecific.android.js

   NativeReactModule.dismissContextualMenu(screenInstanceID);
 }
 
+function showOverlay(params) {
+  savePassProps(params);
+  NativeReactModule.showOverlay(params);
+}
+
 module.exports = {
   startApp,
   push,
   setBottomTabBadgeByIndex,
   showSnackbar,
   showContextualMenu,
-  dismissContextualMenu
+  dismissContextualMenu,
+  showOverlay
 };