Add support for week function on PostgreSQL backend

[andyquinterom]

Add support for week function to be translated in PostgreSQL backend. This should be fairly easy to do since PostgreSQL has a native implementation of this.

As specified in Postgres documentation, we can extract the week from a Date column with the following:

SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 7

We can easily implement this directly into the backend.

[mgirlich]

I added a PR for this. Note that SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40'); is not the equivalent of week() but of isoweek().