TodoList - Swipe left shows priority options

When user does swipe left gesture, we now display the same 3 circle
buttons (those controlling the priority that are in the header) and now
display a copy of them next to the item.

- Copied the code to create the priority circle buttons from the
TableViewController and put them into a new showCircleButtons method in
the Cell class.

- Did the inverse in another new method hideCircleButtons

- Added some cute animations using the MOVE class for when the buttons
appear and disappear.