Support fields date-added and date-modified

[koppor]

I'm working with a bibtex file, which has following lines

  date-added =    {2014-02-17 12:06:22 +0000},
  date-modified = {2015-06-02 20:04:28 +0000},

I currently don't know, which software writes these data fields. I think, however, that we should migrate from timestamp to date-* as the semantics of timestamp is not clear at first sight: Is it the timestamp of the last modification or the timestamp of the creation? Furthermore: Why is no time included? Furthermore: Why is no ISO date used?

[tobiasdiez]

Duplicate of JabRef#130