28byj-48 ball bearing rotator (remix)

This is a remix of http://www.thingiverse.com/thing:1221475

Changes:
- Lower part can now be fixed with screws (can still be used with original upper part)
- Lower part now has a small "deflector shield" to prevent that the cables get hit by screws
- Upper part was changed to work with smaller bearings (624z I got from Aliexpress)
- Some clearance added to holes of the upper part
- FreeCAD sources included