GUI

GUI changes
Merge pull request #4 from ellina-s/ellina

Ellina
Populated Tables for Testing
Merge pull request #5 from ellina-s/arjan

Populated Tables for Testing
Add Item for Managers

Basic GUI for add items
Merge pull request #7 from ellina-s/clark

Clark
Merge branch 'master' of https://github.com/ellina-s/cs304 into ellina
Implemented Customer Registration and added test for it