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/Validation/ArgumentValidator.php

@@ -29,9 +29,9 @@ class ArgumentValidator
         ) {
             //Throw an Exception for string or array
             throw new \InvalidArgumentException("$argumentName cannot be null or empty");
-        } elseif ($argument == null) {
+        } elseif ($argument === null) {
             //Generic Exception
             throw new \InvalidArgumentException("$argumentName cannot be null");
         }
     }
-} 
+}