Use the system python to run update script when running under cygwin.

The win32 python shipped by chromium needs to be fetched by gclient sync,
which then requires running the update script, this gets us into the
problem of chicken and egg.

So instead we run the update script with system python, and only switch
to use win32 python when necessary.
Use list to compare sys.platform.
Use chromium'python to execute chromium's scripts.
Merge pull request #22 from brightray/cygwin2

Improve cygwin support