ObjectNotFoundException has been added

* ObjectNotFoundException has been added
* JsonAdapters throw it in case then there is no such object in database
Create requests.md
Update requests.md
Update requests.md
Required fields support has been added

* fixed problem related to unhandled provided null-values
* toString methods have been added to request pojos
UserSessions' classes have been moved into Users' classes
registerCustomer and editCustomer related bugs have been fixed

* PersistentObject#save() call has been added to the end of PersistentCustomer#PersistentCustomer(...)
* PersistentObject#update() calls have been added to the end of PersistentCustomer's setters
Tests have been fixed
Hibernate session related bug has been fixed

* a lot of changes related to session obtaining have been introduced
Backend:
* customerByName predicate
* customerAutocomplete predicate
* PersistentCustomer bugfix
admin-terminal:
* Still working on "New Request" form
* Customer model implemented
* Customer form implemented with customer and address autocomplete
Still working on "Register new task"...