Enable the embedded mode by default.

The embedded mode is our focus now so that all web apps should be
packaged with the embedded mode by default.

BUG=https://crosswalk-project.org/jira/browse/XWALK-547
Merge pull request #42 from yongsheng/embedded

Enable the embedded mode by default.
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