Fix Eraser

[RogerAK]

This is a failure

• [x] Eraser erases the canvas (i.e the background is light green instead of white)

• [ ] If you erase multiple objects in a the canvas with non continuous strokes and if you click undo, it undoes one erase action and then shrinks all other erase actions.

• [x] The eraser tool is a rectangle eraser instead of a circle.

Before undo:

After undo:

[kbroschke]

Should be worked on using TDD

[kbroschke]

(also see #87 )