update get_version() to follow pep440 to generate version string
catch invalid string exception, and fall back to RELEASE-VERSION
add mocking test for get_version and fix misc issues based on previous review
add functionality to test arugments for each mocked git call
addressing code review; fixing two bugs related with git commands
fix one comment
fix a comment
fix a small issue
fix an import error for pytesting gitpy_versioning as a submodule