Allow content types to define an access callback for create/add

I would like to give authors access to add panes to Panels, but need to be able to restrict certain content types per role.

This patch adds a new 'create content access' key that can be used to add a callback.

The callback will get the string 'create' and the content type ID. The 'create' string is so that it can be passed to most generic access checkers, that expect an $op.