Take null into account when building conditions

(Related with issue #39)

Hi again, 

Today trying to do a query based on a null condition I noticed that aside from packing null as "null" other changes need to be made so that everything is balanced. I mean using "null" as a trigger for the 'is' operator in mysql.

So aside from doing this kind of query ```boss_db:find(module, [{field, 'equals', undefined}])``` this could be used ```boss_db:find(module, [{field, 'equals', null}])```