Changes to ep_update_exclusions() and improved capability check.

* Reverting the removal of the capabilities check in the update_exclusions method by adding it back in a way which respects post_type
* Added an extra check to avoid saving anything on initial creation of a post (was causing errors to display in dashboard quick post box)
* Changes to exclusion status were not being saved when working on a revision. Changed to ensure they *will* be saved, but with the proper post ID *not* the revision post ID.

* Added capability check to ep_admin_quickedit_js() so as to only add the quick edit javascript when the user has the required capability based on post_type.

* Added capability check to ep_admin_init() so as to only add the metaboxes when the user has the required capability based on post_type.