Add icon to executables

Added SetIcon method to Freezer to set the executable icon on windows.

The winexeutil module uses win32api so pywin32 must be installed to be able to use. The module uses the windows api to update resources on the executables. For now it only edits the icons... but in the future it may modify other resources.

Also added a test which is basically similar to test_guiscript.py but adds an icon.