Clock Kit Case (3.5mm barrel plug)

Some variants of the C51 4-bit clock kit use a 3.5mm DC barrel jack for its power source instead of a screw terminal block. For example: http://www.icstation.com/bits-digital-electronic-clock-at89c2051-chip-kits-soldering-practice-learning-kits-module-p-1765.html

This is a remix of [mouseas](https://www.thingiverse.com/mouseas)'s [Clock Kit Case](https://www.thingiverse.com/thing:969988) (v4) with the following differences:
* hole added for 3.5mm DC barrel plug
* buttons integrated into top