Release 0.2.1

With thanks to all the contributors, particularly @codeTom and @boykovdn, we have a new release. This brings Python 3 support, provision for limit switches, improved reliability (fixed an integer overflow), and better handling of optional features. The arduino files also work with the latest and greatest Sangaboard, v3 (not yet uploaded). This is scheduled to be the last release from GitHub, we will be refactoring and moving to GitLab in the near future.