Fix the issue for make_webapp.py failed to run on Windows.

get_xwalk_app_template.py:
1. Change the open flag to 'wb' which will write content of downloaded zip file
    with binary mode.
2. Change some path generation issue, replace to use os.path.join.
3. There is no zipfile.ZipError class on windows, so replace with general exception.
make_webapp.py:
1. Add shell=True flag when need to call external command with subprocess.Popen.
Add the link [Developing on Windows] in README