Implement date trunc

[AlpAribal]

My stab at closing #27.

Opted to always return a datetime without checking/evaluating date, which enables the users to also pass a column name to it.

For tests, I did not use/inherit from BaseDateTrunc because it casts the result as date, which fails for sqlite. But I made sure to include tests for different edge cases and truncating for each datepart.

[AlpAribal]

@codeforkjeff @edgarrmondragon I do not have the rights to add reviewers, could you help me with that?