Added tests for iexact and icontains lookups.

Refs #165.
Implemented iexact and icontains lookups.

Refs #165.
Added tests for a number of field lookups...

including istartswith, iregex, regex, endswith, and iendswith. Refs #165.
Implemented a number of field lookups, including..

regex, iregex, istartswith, endswith, and iendswith. Refs #165.
Added tests for year, month and day field lookups.

Refs #165.
Implemented year, month and day field lookups.

Refs #165.
Added a test for the isnull field lookup.

Refs #165.
Implement the isnull field lookup.

Closes #165.