Fix: dragging when the mouse is outside of the viewers or browser window.
Fix: When the large image was smaller than the space available for it, the viewport wouldn't draw due to out of bounds sampling of imageObj
New: mousedown inside the viewBox now moves it
New: the sub image is now a configurable rectangle instead of a square.

I also reduced the number of multiplications and divisions by changing viewPortHeight to viewPortSize and making use of the kinetic rectangle's getWidth().