Enabled Billing Plans and Agreements APIs

- Added API Classes, Samples, and Tests
- Updated Functional Tests
- Updated Documentation with new SDK Name
- Updated Few Samples to use newer nicer result page

lib/PayPal/Common/PPModel.php

@@ -173,7 +173,7 @@ class PPModel
                     $this->assignValue($k, $arr);
                 }
             } else {
-                $this->$k =  $v;
+                $this->assignValue($k, $v);
             }
         }
         return $this;