fixed issues in PreUpdateEventArgs due to different changeset structure

fix https://github.com/doctrine/phpcr-odm-documentation/issues/60

this is a rather large refactoring, undoing the copy of the 'originalData' in the changeset structure that added in the development of 1.2.0 to handle recomputing changesets (see https://github.com/doctrine/phpcr-odm/pull/529). this data structure is no longer necessary since we changed the changeset structure to match that of the ORM in order to properly support ``PreUpdateEventArgs`` which already contains the data from the changed fields.

there is still an open question about the behavior of the ORM:
http://www.doctrine-project.org/jira/browse/DDC-3248

but it seems like the behavior will be kept .. so we can merge as is imho but I opened https://github.com/doctrine/common/pull/333 as a followup