Add support for arxiv and doi entries

Updated version that adds arXiv:1209.1234 and doi:10.0.0/XXXX to the reference list. 
For now, the style can be over-ridden by redefining \eprint and \doi in the user's
tex file to be a hyperlink, but this is not done by default.

Following our discussion, this does not use the adsurl field - one additional complication with them is that they can contain '%' characters - eg, an A&A reference will contain A%26A, which makes them difficult to reliably include in LaTeX.