Make the code work on non-Windows OS

[lmwang9527]

Besides the installer is Windows-specific, gui/app.py only works on Windows as the path manipulation code there is Windows-specific. Refactoring gui/app.py will make it possible to run the GUI on non-Windows OS by invoking gui/app.py from Python command line (after R & required R packages are properly set up). Then finding a solution for non-Windows installer would be good to have, but not necessary to use the GUI.

[bstabler]

Thanks for the suggestion. We'll fix this in an upcoming release.

[lmwang9527]

Refactoring gui/app.py is done in pull request #8