SQL demo dataset assumes 'postgres' DB username.

[kfogel]

The SQL-format demo data supplied in data/checkbook_demo_database_for_postgres_db_20130524.zip assumes that the DB user is "postgres". Currently this matches the example user given in the INSTALL.md file, but having a dependency on a particular example user is probably not a good idea in the long run.

One solution would be to templatize the SQL file and doing username substition as an explicit step. This will all become less important once the raw demo data files are the recommended way to get demo data into the system, but we're not there yet, so in the meantime we should make the SQL file as useable as it can be.

(Note that the data/checkbook_drupal.sql file doesn't have this problem as far as I know.)