Add SQL logging to PostgreSQL and SQLite

Changes will write sql statements out to STDOUT to help with debugging.

Sample usage:

```nimrod
import db_postgres

db_postgres.SQL_LOGGING = true

...
Exec(db, sql"create table tbl1(id serial primary key not null, one varchar(10), two smallint)", [])

# [create table tbl1(id serial primary key not null, one varchar(10), two smallint)]
```