Add Items Transaction

Add Items transaction for managers complete
Merge pull request #17 from ellina-s/arjan

Merge
Unfinished Process Order menu

Started on Process Order
Merge pull request #22 from ellina-s/clark

Clark
Noticed that manager can reduce stock to negative.

If manager tries to reduce stock it is okay.  If it turns out that stock
will go below zero just set the stock to 0.