Dropping Python 2.4 support

Hello,

Matthew Woodcraft and me have worked on exposing PQresultErrorField on
the Error object, allowing to get detailed information from a server
error. This includes the new diagnostics fields to be released in
PostgreSQL 9.3.

Ticket: http://psycopg.lighthouseapp.com/projects/62710-psycopg/tickets/149
Docs:
  http://initd.org/psycopg/docs/module.html#psycopg2.Error.diag
  http://initd.org/psycopg/docs/extensions.html#psycopg2.extensions.Diagnostics

The feature needs the PQresult stored into Error, making it a
StandardError subclass with a customized C structure. Unfortunately
this breaks Python 2.4, where the exceptions were old style classes.

Working around the issue seems a messy matter of #ifdefs scattered all
around the code: I think it would be better to just pull the plug on
Python 2.4 for psycopg release 2.5. If there was a 2.4.7 release (it
could be: there is a couple of bug fixed and, sigh, another Zope
problem) it would still be compatible with Python 2.4.

Issues? Objections? Let me know.

Cheers,

-- Daniele