Implemented methods in ProductHandler

Implemented methods in ProductHandler and added Javadoc to methods in
ProductContainer.
Added basic Inventory interface
Implemented methods in UserHandler

Implemented methods in UserHandler and changed a few things in User and
UserContainer.
Removed a few comments
Merge branch 'linnea'
Merge branch 'master' into oskar
Small code fixes
Merge branch 'linnea'
Merge remote-tracking branch 'origin/master' into andreas