Log Message:
Update to 0.81

Upstream changes:
Version 0.81 (2013-04-08)
 * Correct reverseName() for IPv6 addresses, so IP('::1').reverseName() returns \ 
correct.
 * Add network mask awareness to v46map()
 * Fix Python 3 errors in IPSet class
 * Make IPSet base class be object when MutableSet isn't available, fixing
   errors in Python 2.5

Version 0.80 (2013-03-26)
------------

 * Drop support of Python older than 2.4
 * Python 3 does not need 2to3 conversion anymore (same code base)
 * Fix adding of non-adjacent networks:
   192.168.0.0/24 + 192.168.255.0/24 made 192.168.0.0/23
 * Fix adding networks that don't create a valid subnet:
   192.168.1.0/24 + 192.168.2.0/24 made 192.168.1.0/23
 * Fix adding with an IPv6 address where .int() was < 32 bits made IPy believe it
  was an IPv4 address:
  ::ffff:0/112 + ::1:0:0/112 made 255.255.0.0/111
 * Add support of IPSets
 * Add support for subtracting a network range
 * Prevent IPv4 and IPv6 ranges from saying they contain each other
 * Add a .v46map() method to convert mapped address ranges
   such as IP('::ffff:192.168.1.1'); RFC 4291
 * Change sort order to more natural:
   IPv4 before IPv6; less-specific prefixes first (/0 before /32)

Version 0.76 (2013-03-19)
-------------------------

 * ip == other and ip != other doesn't fail with an exception anymore if other
  is not a IP object
 * Add IP.get_mac() method: get the 802.3 MAC address from IPv6 RFC 2464
  address.
 * Fix IP('::/0')[0]: return an IPv6 instead of an IPv4 address