JDBC connection can use classpath

If claspath was not set, passed NULL into RJDBC adapter.
Now check for NULL and pass instead empty string (the default option)

When no classpath is passed, JDBC uses the loaded classpaths
Added Heroku source to sql.readers

Heroku is a populat PaaS that offers hosted PostgreSQL databases
unfortunately there is a problem with RPostgreSQL that prevents connecting using the PostgreSQL adapter
It is possible connect using the JDBC adapter, but this requires explict configuration in the URL

This datasource supplies the specialised configuration on top of the JDBC adapter
making the *.sql file look much more like the configuration on the Heroku web site