Automatically convert datetime to date.

Currently `is_working_day()` doesn't respond correctly if you give it a `datetime` object as opposed to just a `date` object. It correctly reports weekends, but not holidays:
```
>>> uk.is_working_day(datetime.datetime(2014, 4, 21))
True
>>> uk.is_working_day(datetime.datetime(2014, 4, 21).date())
False
```