fixed issue with serializing datetime.time(0)

If you try to serialize a `datetime` time object with a value of "00:00:00" you end up with `colander.null` as `bool(datetime.time(0))` evaluates to `False`.  I moved the if statement so that it first tests if we have a `datetime.time` object and then if we don't, evaluate if it's a `False` value.  So, now you can properly serialize a `datetime.time(0)` time.

I'm not really sure why Python implements a `__bool__` on `datetime.time` or why "00:00:00" is considered False.

Also fixed a minor documentation bug.