Enclosure with countersunk screw option

Made a couple of minor tweaks:
* Modified the scad file so I could use countersunk M3 screws for the cover
* Enlarged the holes a little so that the screws go in a little more easily
* Sliced and printed the enclosure without the top and bottom layers (just had the infill) for better ventilation for the contents. Will reprint the cover in a similar way for better airflow.