You can clone with
HTTPS or Subversion.
Most of the core plugins are using Piwik_Version::VERSION for their version number, but not so for the example plugins.
The version number policy is:
A policy for transitioning 3rd party plugins to core will be formalized after #1116.
(In ) fixes #1222 - document the version number policy; point developers (who read the code) to Piwik_Plugin (core/Plugin.php)