Delete tries to delete this entry from its feed. If the entry
does not contain a link rel="edit", we throw an error (either
the entry does not yet exist or this is not an editable
feed). If we have a link rel="edit", we do the empty-body
HTTP DELETE to that URI and check for a response of 2xx.
Usually the response would be 204 No Content, but the Atom
Publishing Protocol permits it to be 200 OK.
Save is used to either create new entries or to save changes to existing
ones. If we have a link rel="edit", we are changing an existing entry. In
this case we re-serialize the entry and PUT it to the edit URI, checking
for a 200 OK result.
For posting new entries, you must specify the $postUri parameter to
save() to tell the object where to post itself. We use $postUri and POST
the serialized entry there, checking for a 201 Created response. If the
insert is successful, we then parse the response from the POST to get any
values that the server has generated: an id, an updated time, and its new
link rel="edit".