Hex shaft options

Alex:
   Here is the pull request for the change.   The part moving the set screw by 30 degrees could likely be applied to the round shafts as well without issue.   Feel free to use my full name David Boyd  in any docs.   Thanks for a great library.  Here are some pictures of items rendered by openscad with the hex shaft and also a printed example of using the library to create a spacers.
![hexspacer_1inch](https://f.cloud.github.com/assets/3876300/1905772/58ceff74-7ca3-11e3-9c16-60d07fd0dd25.png)
![hexborewheel](https://f.cloud.github.com/assets/3876300/1905773/58d2613c-7ca3-11e3-9ffb-c65129698e65.png)
![2014-01-13 07 39 24](https://f.cloud.github.com/assets/3876300/1905791/669c1bd2-7ca3-11e3-91c1-a303ee136e04.jpg)