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/lib/main.dart

@@ -0,0 +1,24 @@
+import 'package:flutter/material.dart';
+import 'place_map.dart';
+import 'package:google_maps_flutter/google_maps_flutter.dart';
+
+class _Home extends StatelessWidget {
+  const _Home({ Key key }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return PlaceMap(
+      center: const LatLng(45.521563, -122.677433),
+    );
+  }
+}
+
+void main() {
+  runApp(
+    MaterialApp(
+      debugShowCheckedModeBanner: false,
+      title: 'Place Tracker',
+      home: _Home(),
+    )
+  );
+}