Added Heroku source to sql.readers

Heroku is a postgres DB but doesn't play nice with the RPostgreSQL package. Instead requires custom settings using a JDBC connection. I find it a pain to copy all of those custom settings into a .sql file with type = jdbc. So created a wrapper with type = heroku. Kind of related to the philosophical questions raised in PR #18, but since this is a wrapper for an already supported reader I don't think it is as big of a potential problem as adding a completely new reader type.