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/Core/PPCredentialManager.php

@@ -144,7 +144,7 @@ class PPCredentialManager
      */
     public function getCredentialObject($userId = null)
     {
-        if ($userId == null) {
+        if ($userId == null && array_key_exists($this->defaultAccountName, $this->credentialHashmap)) {
             $credObj = $this->credentialHashmap[$this->defaultAccountName];
         } else if (array_key_exists($userId, $this->credentialHashmap)) {
             $credObj = $this->credentialHashmap[$userId];