Outdated usage sample in README

README has this example:
```
>>> from calendra.europe import France
>>> cal = France()
>>> cal.holidays(2012)
[(datetime.date(2012, 1, 1), 'New year'),
 (datetime.date(2012, 4, 9), 'Easter Monday'),
 (datetime.date(2012, 5, 1), 'Labour Day'),
 ...
 (datetime.date(2012, 12, 25), 'Christmas')]
```

But `holidays()` returns a list of `Holiday` objects, not tuples.