harvest DOIs from arxiv

[annetteh]

DOIs should be harvested from arxiv which has a dedicated doi field.
example:
​http://inspirebeta.net/record/913556
​http://arxiv.org/abs/arXiv:1106.2519
Journal reference: Nuclear Physics B (Proc. Suppl.) 214 (2011) 137--142
DOI: 10.1016/j.nuclphysbps.2011.03.073

[jcaffaro]

This should be addressed (for the sample oaiarxiv2marcxml.xsl conversion style that ships with Invenio) as part of branch master-bibconvert-oai-arxiv-conversion-fix