In World.js, changed getAttributes() back so that it returns a list of
just the identities of the attributes, rather than attribute items.
Added a new method, getAttributeItems(), to return the items themselves.
Fixed plugins accordingly. This change helps ensure that the
OpenRecord code is using the dojo.data APIs in a "pure" way, which
hopefully should make the OpenRecord more readable to anyone who's
coming to OpenRecord from a dojo background.