Store next IDs in config file

A config.properties file gets created and initialized now along with
the “DB CREATE” command.  Likewise, the file gets deleted on a “DB
DROP”.

I changed my InvoieManagerImpl to use the new db.getNextID() method, we
need to do the same for Warehouse and Truck now also.
Author tags and iteration markers
Use case: Employee queries for invoice by state
Use case: Mark package delivered and customer notified
Merge pull request #6 from aguibert/master

Pull Andy