Add ProCite support to the RIS translator

Merging in most of the changes from https://forums.zotero.org/discussion/22615/procite-to-zotero-conversion-translator-ris-and-testing/

Also fixes https://forums.zotero.org/discussion/33503/series-volume/ (edit: well, partially. Need to fix this properly before merging)

Test data (from the ProCite thread) with ~5000 entries is available here: https://www.dropbox.com/s/mror411719066d2/procite%20export.txt

This doesn't change anything for EndNote RIS import (except for the series number) and I didn't notice an increase in import times.

Any thoughts on this? I'll squash it once it's ready to be merged (though it seems to be working ok now).