update version-check message in setup.py and IPython.__init__

clearer message that IPython no longer supports 2.6.
update Python version classifiers:

- drop specific 3.x
- drop 2.6
update README with dropped 2.6 and 3.2 support

also clarify the checkout steps in development install
Merge pull request #4190 from minrk/byebye2.6

update version-check message in setup.py and IPython.__init__

clearer message that IPython no longer supports 2.6.