"New style" arXiv preprints should have extra fields

[Micha Moskovic]

When an arXiv preprint is added, only the "eprint" field is added to the document.
However, the arXiv changed its identifiers in 2007, and for those new identifiers it would be nice if the fields "archivePrefix" and "documentClass" were added too, because they are used by some BiBTeX styles. See http://arxiv.org/hypertex/bibstyles/ for more details.