Documentation Updates and Minor Refactoring

- Updated README and Release Notes
- Updated PayPal Constant to reflect latest release

lib/PayPal/Common/PayPalModel.php

@@ -14,6 +14,24 @@ class PayPalModel
 
     private $_propMap = array();
 
+    /**
+     * OAuth Credentials to use for this call
+     *
+     * @var \PayPal\Auth\OAuthTokenCredential $credential
+     */
+    protected static $credential;
+
+    /**
+     * Sets Credential
+     *
+     * @deprecated Pass ApiContext to create/get methods instead
+     * @param \PayPal\Auth\OAuthTokenCredential $credential
+     */
+    public static function setCredential($credential)
+    {
+        self::$credential = $credential;
+    }
+
     /**
      * Default Constructor
      *