new dataobj and dataset classes (unfortunately I cannot cut down all the various features being introduced):
- support for generic flows (formerly eprint_status = inbox, archive etc.)
- enhanced ACL-like checks (support flows, security contexts etc)
- generalise internal/system fields and concepts: history, revision, datestamp, lastmod
- adds a few new dataset properties: read-only, flows, acl
- dataobj: possibility to revert to a former revision number
- dataset/dataobj: support for memcached (caching of data objects) if enabled
note that some clean-ups must still be done. those are marked as 'TODO' and 'sf2'