added methods series and seriesaddedentries

New methods to get series fields (440 and 490) and series added entries (8XX) from bib recs.

i.e.

reader = MARCReader(open('JSTOR.mrc'))
for record in reader:
    for series_statement in record.series():
        print series_statement
    for series_entry in record.seriesaddedentries():
        print series_entry