Allow to set order by on search requests, and allow to clear the cache

Hello,
I'm developing an integration with OpenERP. Attached patch does the following:

* Add an $order parameter to the search and page methods. This accepts an order by clause as expected by OpenERP, so e.g. name ASC or parent_id ASC, id DESC.

* Allow to clear the internal static cache (I needed this when updating a record that is about to be accessed again in the same request)