Fix adding timestamp in ccm_addHeaderItem

When adding a timestamp to the resources (.css/.js files) loaded by `ccm_addHeaderItem`, a timestamp is added to avoid cache.
BTW, if the URL of the resource already contains a GET parameters (eg `file.js?a=X`), the timestamp is appended withoud adding a new parameter (eg `file.js?a=X1393878504000`).
That will mess up the existing GET parameter.

Let's fix this by adding a new GET parameter when adding the timestamp even when we already have other GET parameters.