* Fixed service - provider connection in backend/calendar appointment modal.
* Backend notification messages on top are now disappearing after a few seconds (excluding cases where there are action items - the user must close the notification)
* The user will be able to select an existing customer from the backend/calendar appointment modal when creating a new appointment record.
* Started work on user privileges and on how the system performs according to that.
* Admins can manage the working plan of a single provider on the backend/users page.
* Before sending a new notification email the system checks whether the provider user has notifications enabled.
* Added salt field in the "ea_user_settings" table.
* Regenerated code docs (js + php).
* Added custom error 404 page.
* Added service price and description in the appointment booking wizard.
* Completed Issue #21 - devide services in categories in the appointment booking wizard.
* Completed the backend user management page.
* Fixed and refactored existing code.
* Updated database structure (appointment foreign keys need to be null, when the record is a unavailable time period).
