Adding PostgresSQL support.

Hi, 
I am in the process of adding postgresSQL support for hyena. I would like to know your suggestions on following issues
- Handling different syntax of both dbs. Should we use any query builders such as Knex?
- Running same tests for both engines.

I just completed writing `lib/pgsql.js`  using `generic-pool` and `pg` modules. Now got I stuck at these points.
