to allow usage as a package from external golang projects.

Full changes:
- enforce TLS and full validation
- made the code more testable
- made it usable as a package (split the main to the cmd folder)
- added end to end main unit test that launches a server and client
- added Makefiles for common dev tasks
- added cert generation for testing
- refactored the config to provide defaults
- moved config validation to the respective server and client code paths
- added a bugfix to fix a crash when total duration <= 2 * update
  interval
- changed docs to reflect the changes
- moved the version to be an exported package property in config.go

TLS enforcement and package refactoring