Finished the addItem for Manager Transaction

In the project description it says it adds existing items so I assumed
the quantity is the number you want to add to the current stock.
Merge pull request #11 from ellina-s/arjan

Finished the addItem for Manager Transaction
Merge pull request #12 from ellina-s/clark

Customer Registration
Merge pull request #13 from ellina-s/clark

Sales Report Transaction