Add Google Maps Place Tracker sample app to flutter/samples. (#20)

Sample app that displays places on a map. Add/edit places. Interact with map. Iterations to follow.

place_tracker/ios/Runner/main.m

@@ -0,0 +1,9 @@
+#import <Flutter/Flutter.h>
+#import <UIKit/UIKit.h>
+#import "AppDelegate.h"
+
+int main(int argc, char* argv[]) {
+  @autoreleasepool {
+    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
+  }
+}