Refactored code and added commit method

- All loadMethods have been refactored
- commit method has been added to class
- Deleted ArrayList deletedCustomers
- Deleted registerDeletedCustomer method

Reviewed by: Christoper
JavaDocs made, class refactored

- All methods have been refactored
- All methods ready to use, except findBookingByParams
- JavaDocs created for all methods

Reviewed by: Christopher