Add hook_civicrm_coreResourceList

After working on CRM-16354 and CRM-16444 and an ensuing discussion on the blog, it seems clear to me that we need this hook. Granted, CRM_Core_Resources::coreResourceList is not "the best" of functions and may eventually get supplanted by a more efficient dependency-loader. But for now, extensions like TinyMCE need a reliable way to alter that list. Since there's already a quasi-hook in place for the benefit of cms-integration code, we might as well make it a real hook.

Since 4.6 is the next lts release, seems to make sense to go ahead and put it there, similar to the way new apis get backported to the lts.