Fixed folder handling for windows with spaces in filepath

In windows the path handling will not work correctly if we use ```env={"PATH": path}```. By copying the os environment with ```os.environ.copy()```, and modifying the *PATH* if ```exec_args``` contains ```path``` and use this _env_ as input to the ```subprocess.Popen``` I have testet the changes with success on both Windows 7 and Ubuntu 13.10.