Python 2.5 compatibility.

A small tweak to the code to work woth Python 2.5 and an addition to the
README to state what Python versions are compatible. (I assume 2.4 and
earlier are not compatible.)
Merge pull request #66 from jim-easterbrook/master

Python 2.5 compatibility.
Update README.md
Update CHANGES
Update setup.py
Update twitter.py