allow PYTHON_CMD or GIT_CMD to be set from the environment

and use more elaborate command availability checks
fix typo in message "repsitory" -> "repository"
Merge pull request #13 from ploxiln/flexible_python_13

allow PYTHON_CMD or GIT_CMD to be set from the environment
fix var name
Merge pull request #15 from mrwoof/15_var_case

$issue_number -> $ISSUE_NUMBER