Added "is null" and "is not null" comparators for Datetimes.

They work a little bit different from other comparisons, since "HTML parameters don't usually work with a language-specific construct of nil". The passed in value for the filter is ignored and the comparison to nil/NULL is made instead.

For example, if you are filtering by parameters, you could 'add my_column_is_null' to the query string without a value - any value specified will be ignored.

This addresses https://github.com/rcdilorenzo/filtrex/issues/62. It only adds the null comparators for DateTimes, however, it is straightforward to add them to other data types.