First round of extra credit bits

- removed cell background colors
- using cell text color for complete/overdue status instead
- also using strikethru text for completed tasks in tableView
- date displays initially in textField
- tapping field toggles datePicker in place of the keyboard
- textField & textView hilites when receiving focus in Add and Edit views
- made the layout consistent on all views. Removed borders in Detail view.
- coded placeholder text in detailTextView in the Add VC
since there's no option for that in the storyboard as with TextFields