Schoolstuff

### This is an entry for the #backtoschool Challenge

Objectives:

* good functionality
* no sharp edges
* sufficient sturdiness
* use of a moderate amount of material
* acceptably nice look

### Ruler:
Since compared to 2D printers 3D printers have limited resolution it is intened to print the scale on paper or transparent foil and glue it onto the rulers edge with scotch tape. I've included a ruler vectorgraphic file (easily editable with inkscape). The rounded end