Fixes #14 - Finding 7Zip Executable

get_7za_bin() now does a more thorough search for the 7zip executable
looking for it in both "Program Files" and "Program Files (x86)"
directories.

The settings.get('7za_path') function isn't work for me? I have edited
the User/Sublimall.sublime-setting file, still it return None. WTF?