Adds the database abstraction layer (DAL) with a Transactions method
and a Connection object to encapsulate the DB open connection.
Includes a DAL unit test using the go-sqlmock library.
Connection to a PostgreSQL database using environment variables to
store the secrets.
Overall, with the /transactions endpoint requesting directly to the DB,
this patch finish the tracer bullet project.
Issues #1, #5, #6 and #7
