(COMPOSER) Datetime pickers
See: https://github.com/kartik-v/yii2-widget-datepicker
https://github.com/kartik-v/yii2-widget-timepicker
https://github.com/kartik-v/yii2-widget-datetimepicker
[#14] Better defined validation
TimeSlotModel creation view
Fixed date validation bug
(MIGRATION) Added 'generated_until' field to TimeSlotModel
(MIGRATION) Changed type of 'frequency' in table TimeSlotModel to adopt ISO8601: http://en.wikipedia.org/wiki/ISO_8601#Durations
Adapting to the changed type of frequency
First draft of spawnTimeSlots method
Refactoring method using coding conventions
rename directory timeslot
update in timeslot creation
Updated and fixed timeslots validations
update timeslot
Merge branch 'timeslotmodel' of https://github.com/DSD-BookEasy/BookEasy into timeslotmodel

Conflicts:
	models/Timeslot.php
finisched timeslot creation
fixed stuff in timeslot and timeslotmodel
unbinding of model on update timeslot
Removing validation because it isn't working as expected
Refactoring of TimeSlotModel generation of TimeSlots for clarity and code reuse
Added a method to delete TimeSlots that were generated from the TimeSlotModel