Add class aliases for Carbon.

This should help make switching libraries even easier.

README.md

@@ -36,9 +36,11 @@ First add `cakephp/chronos` to your `composer.json`:
 php composer.phar require cakephp/chronos
 ```
 
-To update your code you will need to update imports and typehints. Assuming
-`src` contains the files you want to migrate, we could use the following to
-update files:
+By default Chronos includes a compatiblity script that creates aliases for the
+relevant Carbon classes.  This will let most applications upgrade with very
+little effort. However, if you'd like to permanently update your code,you will
+need to update imports and typehints. Assuming `src` contains the files you
+want to migrate, we could use the following to update files:
 
 ```
 # Replace imports

composer.json

@@ -31,7 +31,8 @@
   "autoload": {
     "psr-4": {
       "Cake\\Chronos\\": "src"
-    }
+    },
+    "files": ["src/carbon_compat.php"]
   },
   "autoload-dev": {
     "psr-4": {

src/carbon_compat.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @copyright     Copyright (c) Brian Nesbitt <brian@nesbot.com>
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+
+// Create class aliases for Carbon so applications
+// can upgrade more easily.
+class_alias('Cake\Chronos\Chronos', 'Carbon\Carbon');
+class_alias('Cake\Chronos\ChronosInterface', 'Carbon\CarbonInterface');