Filter using objects, not just strings.

Previously, we were sending qs.filter() only string values (excepting the simple values True, False and None).

This fixes #385 for me.  #385 is an issue with string-value based date filtering, not necessarily with tastypie.  Previously we would pass filter() something that looked like {'key__gte': '2012-06-25T22:21:30'}.  At least for me, in Django 1.3.0, a filter() using an ISO8601 date string just doesn't work -- it needs to look like '2012-06-25 22:21:30' for it to work.  I'm guessing this is because of django.conf.global_settings.DATETIME_INPUT_FORMATS, which doesn't support ISO8601.

So maybe we should be filtering with a datetime object and not just a string?  I use the associated Field object's convert() method to turn the string into a python object.
IMHO this should have been mvoed to BooleanField.convert a long time ago.
If you want to filter `__isnull=True` or similar then handle operator expression somewhere else because this is `value_to_python`. And then  in convert just check isinstance(value, (str, unicode)): value.lower() == 'true' and the same for false.