Publication Years (PY) are not imported from RIS file

Hello. Thank you for the excellent module. We recently used Biblio to import a number of citations in RIS format. Everything came across except for the year of publication. An examination of our RIS data shows the year is labeled as 'PY', whereas the ris_parser.inc is looking for 'Y1'. The attached patch adds a 'PY' case for the year field.