Gimbals with letter

Gimbals with a letter in the middle.

All letters are more or less fitted and centered, but you may want to adjust them further using the `.scad` and OpenSCAD.

TODO:
* allow more than 4 rings
* fourth ring a little loose
* allow attaching gimbal sets together, or a gimbals stand

See also:
http://www.thingiverse.com/make:90021
http://designmaketeach.com/2014/05/06/3d-design-personalize-a-gimbal/