Add test of exec_cmd
Add wrapper for subprocess.Popen, exec_cmd
Ignore swap files and .DS_Store
Add test for gem_probe and make it pass
Refactor gem_probe
Modify exec_cmd to ignore uninstalled commands
Add test for dpkg_probe and make them pass
Refactor dpkg_probe
Add current working directory param to exec_cmd
Add npm_probe
Adhere to PEP 8 - add blank lines
Move comments to docstring