Enable abstract downloading beyond ADS

[zhafen]

Currently abstracts can only be retrieved for papers on ADS. While fine for astrophysics, this is insufficient for other fields.

Additional Info:

• In many cases the abstract will be retrieved by Mendeley upon downloading the paper, but papers in Nature and Scientific Reports are exceptions.
• On average it takes slightly less than a minute to manually add in the abstract.
• Some papers in some disciplines (e.g. philosophy) do not have abstracts. For these I've substituted in the first paragraph for the moment.

[zhafen]

Any alternate source to ADS must have at minimum:

• abstracts
• publication or arxiv-posting date
• any and all identifying information available
• citation count

Additional currently-used attributes are:

• papers cited by the publication and that cite the publication (used for building citation networks, which is the default way the literature space is built up)

[zhafen]

Leads for alternative databases:

• arXiv (promising, but more technical work required)
• web of science (proprietary, but check to see if we have access)
• pubmed
• APS
• arts and humanities citation index (through web of science)
• other (check bibliometrics, metascience journals)?