* Fixed minor issues with backend.
* Fixed get_provider_available_time_periods() method - now works correctly.
* Added unavailable notes in the calendar (under the "Unavailable" title).
* Updated the thesis code snippets.
* Added listings.pdf, a free ebook about latex listings.
* 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.
* 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.
