Defer Services_Json inclusion/initialization until really needed

Alternatively, I'd consider dropping Services_Json altogether. Since php
5.2 php comes in with bundled json ext so it should be available everywhere

class/serialisation.php

@@ -1,15 +1,15 @@
 <?php
 
-if (!class_exists('Services_JSON', false)) {
-    require_once dirname(__FILE__).'/services_json.php';
-}
-
 if(!function_exists("CS_REST_SERIALISATION_get_available")) {
     function CS_REST_SERIALISATION_get_available($log) { 
         $log->log_message('Getting serialiser', __FUNCTION__, CS_REST_LOG_VERBOSE);
         if(function_exists('json_decode') && function_exists('json_encode')) {
             return new CS_REST_NativeJsonSerialiser($log);
         } else {
+            if (!class_exists('Services_JSON', false)) {
+                require_once dirname(__FILE__).'/services_json.php';
+            }
+
             return new CS_REST_ServicesJsonSerialiser($log);
         }    
     }