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
Merge pull request #20 from johnmyleswhite/heroku

Added Heroku source to sql.readers