[dojo.data] fix callback names in Read API

[skinner]

At our ​IRC meeting on 2007-02-27 we resolved to change the names of the keyword arguments used for specifying callbacks in the dojo.data find/fetch methods. We need to update the doc-comments in the dojo.data.core.Read.js file accordingly. We also need to go through all the existing datastores and unit tests and change the names there.

old name      new name
------------  -----------
onbegin       onBegin
onnext        onItem
oncompleted   onCompleted
onerror       onError

[Adam Peller]

See #2704