Added comments
Aesthetic changes to GUI etc.
Database structure updated. Database EER Diagram and SQL File modified
with these new changes.

- Added relations between some tables (One to many)
- Changed double data type to Decimal()
- Increased lenght of data for username to fit e-mail adresses
 Database structure updated. Database EER Diagram and SQL File modified
with these new changes.

- Added relations between some tables (One to many)
- Changed double data type to Decimal()
- Increased lenght of data for username to fit e-mail adresses