Update the API 'updateTag' to support project_id parameter

I had been looking for a way to understand global tags vs project tags. It seems from inspecting the results of the “getAllTags” API call that global tags simply have a project_id==0. If the “updateTag” method were enhanced to (optionally) support this parameter, then I could write a script to convert project tags to global tags, including walking previously assigned tasks and updating them accordingly (if that proved necessary).