Provide a constant for the sector size.

This is useful so that subclasses don't have to use the same magic
number.
Allow subclasses of ISO9660 to override _get_sector.

Sample usage:

class MyISO9660(ISO9660):
    def __init__(self, url):
        self._get_sector = self.my_get_sector
        super(MyISO9660, self).__init__(url)

    def my_get_sector(self, sector, length):
        ...
Merge pull request #6 from dcoshea/master

Enable ISO9660 to be subclassed with an overridden get_sector method