Refactor version script

The current version script is a bit hacky and calls shell scripts from python.  It should be rewritten to use pure python and better integrated into the build system.

<major>.<minor/tag>.<commit>.branch_name

This version should be returned when:

*  `cebl --version`
*  in the bottom right corner of the GUI
*  in the file name of builds
*  in the latest git tag