Support more schemes for PDF URIs in BibTeX Import

[karnesky]

Many programs give links to PDFs in their BibTeX export. Unfortunately, it is highly non-standard. We only support:

Old JabRef

pdf={/full/path/to/file.pdf}

but the full path is sometimes prefixed by the protocol (Zotero assumes that it is absent & prepends '​file://'). Further, it is often stored in other fields:

Sente:

sentelink = {file://localhost/full/path/to/file.pdf,Sente,PDF}

New JabRef:

file = {Description of the file:/full/path/to/file.pdf:PDF}

and probably many others.

See:
​http://forums.zotero.org/discussion/3365/
​http://forums.zotero.org/discussion/3568/
​http://forums.zotero.org/discussion/4264/