Compare abstract before replacing publication.

Hi Jonathan,

Here's a new patch to compare abstracts (besides author list) when searching for older versions of publications. The old algorithm thinks eg, that both

http://adsabs.harvard.edu/abs/2013arXiv1304.4719C
http://adsabs.harvard.edu/abs/2013arXiv1304.4720C

are the same publication, and thus removes the first one and adding the second. I tested as best as I can (it should be fine for entries without abstract), but it's hard to test all the possible configurations...