birdnotes.py: Objects to represent bird note files

Matches the name of a month file.

YYYY_MM_XML_PAT = re.compile(
    r'[12]'          # Matches 1 or 2
    r'\d{3}'         # Matches three digits
    r'\-'            # Matches a hyphen
    r'[01]'          # Matches 0 or 1
    r'\d'            # Matches any digit
    r'\.xml'         # Matches a period followed by 'xml'
    r'$')            # End anchor, insure a full match