Remove code in viewDidLoad

The functionality will be replaced by CoreData.
Added CoreData framework

Added CoreData framework in ‘frameworks’ folder.
Setting up the CoreData stack

Added code for CoreData that would automatically be added if we
selected CoreData when creating a new project.
Added Data Model

named TaskItModel
Match Data Model name in CoreData

URLForResource should have the same name as assigned for our Data Model.
Created TaskModel Entity
Removed TaskModel.swift

Since we created our TaskModel entity, we no longer need our
TaskModel.swift
Created NSManagedObject subclass
Created an Objective-C bridge

This enables us to interact with Obj-C code.
Created instance of TaskModel

Added code in addTaskButtonPressed to create a TaskModel instance and
saveContext.
Use a fetchRequest for verifcation

To make sure adding the new task worked, we can use a fetchRequest to
view all of the items from coreData.
Setting up the FetchedResultsController
Updated UITableView DataSource functions

Updated functions in the ViewController.swift
Updated prepareForSegue

Also removed mainViewController property from TaskDetailViewController
and AddTaskViewController.
Updated TaskDetailViewController

Refactored code for DoneButtonPressed
Added Completion Sort Descriptor