Fix typespecs, related to date and time. Fixes #60.

[seriyps]

Actualize date and time typespecs.
The problem is that erlang calendar and postgresql timestamp have a bit different data ranges, plus, epgsql returns time seconds as floats.

https://www.postgresql.org/docs/current/static/datatype-datetime.html

Fixes #60

[seriyps]

So, since there is no opposition to this change, I think it could be merged.

[seriyps]

Thanks!