Updated readme: Python >= 3.2

I tried to run the code (Python 3.1.2) and got this error:

```
Traceback (most recent call last):
  File "ranwhen.py", line 173, in <module>
    half_hours_in_day = round(day / half_hour)
TypeError: unsupported operand type(s) for /: 'datetime.timedelta' and 'datetime.timedelta'
```

Python 3.2 [added support](http://docs.python.org/3.2/library/datetime.html) for division between `timedelta`, so the I updated the readme file.