Branch/drawing rect

@eric79 @youngca 
The text, image, and sprite drawing objects all now have a border.

Also, fixed the dragging of icons so that the dragging is relative to the last move.
One thing that I learned while optimizing `mars-game` was that string concatenation is actually slow.  It does a new memory allocation for every `+`.  A faster way is to do a `join` on an array of strings as described [here](https://developers.google.com/speed/articles/optimizing-javascript).

I don't particularly mind if you decide to leave this as-is, but I thought I would share what I learned.  :smile: