Only expose meta on 'edit' context

As [raised by iandunn](http://wpapiteam.wordpress.com/2014/03/25/custom-post-types-and-privacy-expectations/) and [discussed on IRC](http://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2014-04-15&sort=asc#m832939), we should only expose post meta on the `edit` context.

This is a *temporary* workaround for the permissions issues. We'll come back to this.