Allow moving the screen for letterboxing or screen shake.

This commit allows you to move HXP.screen.x and HXP.screen.y in order to re-position the game screen. HXP.screen and HXP.engine both need to be moved at the same time so that cursor positions and drawing positions are adjusted correctly. I use this for letterboxing to adjust the screen size as the window is resized while maintaining the aspect ratio. It can also be used for a screen shake effect. (I added a screen.shake function to make this easy.)

This also moves the debug console's mask outlines into the correct position. I haven't tested this yet, but I suspect this will fix https://github.com/HaxePunk/HaxePunk/issues/235.

![screenshot - 04112014 - 10 49 06 pm](https://cloud.githubusercontent.com/assets/544977/2686181/9330d694-c1ed-11e3-8c8b-938ac90b3946.png)