Sort metadata keys by priority.

def metadata_sorter(x, y):
    """ Sort metadata keys by priority.
    """
    if x == y:
        return 0
    if x in METADATA_SORTER_FIRST and y in METADATA_SORTER_FIRST:
        return -1 if METADATA_SORTER_FIRST.index(x) < METADATA_SORTER_FIRST.index(y) else 1
    elif x in METADATA_SORTER_FIRST:
        return -1
    elif y in METADATA_SORTER_FIRST:
        return 1
    else:
        if x.startswith('_') and y.startswith('_'):
            return cmp(x[1:], y[1:])
        elif x.startswith('_'):
            return 1
        elif y.startswith('_'):
            return -1
        else:
            return cmp(x, y)