Exit and Draw Command

* shortcuts work along with the menuitem clicking
* **exit** will just plainly exit, no error checking or graceful exiting done yet (doesn't support redo or undo)
* **draw** invokers were hard to do mainly because the receiver itself was an invoker (mouse clicking on canvas). 
* **redo** and **undo** are implemented for **draw**. You can undo as many times as you want and redo as well, as long as you don't perform a new command (i.e. you should not be able to undo->draw->redo, nothing should happen when you try to redo)
* other commands will be implemented when files are done