Only show post meta on edit context

See http://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2014-04-15&sort=asc#m832939
for the discussion around this. This is a temporary workaround for the
data privacy problem and still requires a more permanent fix.
Merge pull request #135 from WP-API/meta-on-edit-context

Only expose meta on 'edit' context as a temporary workaround.  Closes #135.