Removing Dependency from SDK Core Project

- Copied files required for Rest API SDK
- Removed PPApiContext and directly connected APIContext with PPConfigManager
- Removed duplicate data storage of configuration and credentials.
- Code Style Fixes
- Remove build.xml file as it is not required anymore
- Updated the samples
- Updated the documentations

lib/PayPal/Common/PPArrayUtil.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace PayPal\Common;
+
+/**
+ * Class PPArrayUtil
+ * Util Class for Arrays
+ *
+ * @package PayPal\Common
+ */
+class PPArrayUtil
+{
+
+    /**
+     *
+     * @param array $arr
+     * @return true if $arr is an associative array
+     */
+    public static function isAssocArray(array $arr)
+    {
+        foreach ($arr as $k => $v) {
+            if (is_int($k)) {
+                return false;
+            }
+        }
+        return true;
+    }
+}