[PYTHON-CRYPTO] things missing from PEP 272

[Paul Rubin]

   > Also, the IV should be passed as an argument to encrypt() and
   > decrypt(), not to new().

   I should probably explain - reusing IV's is almost always a bad
   idea and one of the common gotchas in protocol design. No need to
   encourage it. Also, no need to force the overhead of another key
   setup just to change IV (not a big deal for AES, but key setup for
   blowfish (for example) is glacial.)

The IV is an instance variable in the crypto object.  You can set it
to a new value without calling new().