Add new endpoints and response attributes.

New endpoints:

 * get_children (for menu building)
 * get_siblings (for menu building)
 * get_parents (for menu building)
 * get_published_draft (for previews)
 * get_draft (for previews)
 * get_recent_drafts (for previews)

New response attributes:
 * menu_order
 * parent
 * absolute_slug: to response since you need the full slug to access it via slug. For instance if you have a parent page "foo" with a child page "bar" you need to access ?slug=foo/bar.

New Features:
 * send login credentials for private blogs

This additions have been live for almost a year.