Customizable involute gear and rack

See the original for full description. This version includes a couple of minor fixes and updates for openscad 2015. Modifications:

- For the sake of the customizer I've set the tooth size to 2*pi mm. This makes the pitch radius equal to the number of gears. If you want different gear teeth sizes you can either download and edit the openscad file or scale the gear in your printer software.
- Added some different hole types including D, and hex for coupling to D-shafts and Hex bolts.
- Adopted a bug fix
- Users can now enter the helix angle rather than the degrees of twist.
- Fixed a bug where helical gears would create bad objects.