Implement the in and !in query operators for PostgreSQL

[dstpierre]

The query operators in and !in which are suppose to look for presence or absence of an item in a list isn't implemented yet in the PostgreSQL database package.

In database/postgresql/query.go line:37, an implementation is required for these query operators to work.

As reference to find the way to implement for PostgreSQL's JSONB format: https://www.postgresql.org/docs/9.5/functions-json.html

This change once implemented can be tested via a test in the database/postgresql package using the QueryDocuments function for instance.