Refactor Directories

Move Engine Files to /src Directory

lib/src/App.dart

@@ -1,7 +1,7 @@
 import 'dart:html' as html;
 
-import 'package:rules_of_living/Cell.dart';
-import 'package:rules_of_living/Grid.dart';
+import 'package:rules_of_living/src/Cell.dart';
+import 'package:rules_of_living/src/Grid.dart';
 
 class App {
   // Elapsed Time Counter - useful for Safety Timeout

lib/src/Cell.dart

@@ -1,4 +1,4 @@
-import 'package:rules_of_living/Rule.dart';
+import 'package:rules_of_living/src/Rule.dart';
 
 class Cell {
   bool state;

lib/src/Grid.dart

@@ -1,7 +1,7 @@
 import 'dart:html' as html;
 
-import 'Cell.dart';
-import 'Rule.dart';
+import 'package:rules_of_living/src/Cell.dart';
+import 'package:rules_of_living/src/Rule.dart';
 
 class Grid {
   final int w;