Datetime filtering

Refs #1338.

I added 3 test cases:
1. Filtering a datetime field by an isoformatted datetime
2. Filtering a date field by an isoformatted date
3. Filtering a date field by an isoformatted datetime

The issue is the way django_filter sets are constructed, because dates compared with an iexact will not be found via django but dates found with an eq will be found even when passed in iso8601 format.  Django_filters defaults to an exact query if no filter type is specified. It seems to me that the best way to handle this is to update the DRF documentation to state that when passing in an isoformat datetime you need to supply the lookup_type='eq'. I'm working on PR that has tests that validate this and will submit it asap.