Add tox.ini file to run tests with Python 2.7 and Python 3.2

I suggest to remove the `install_and_test.sh` script, because you can almost reproduce it with tox.

I didn't changed README yet, but I can add it as a dependency and change the text in "testing" section.

Depends on campadrenalin/DoctestAll#2 to give correct output when tests fail.