web.py: clean up cookiejar stuff in Web c'tor

1. `sync_save` logic was wrong when `cookiejar is not None`

2. Use `StevezillaCookieJar` when no cookiejar is specified, instead of
   whatever the butt python decides to use by default. this way the
   caller can retrieve it after the fact and use it how they please

3. Automatically disable `sync_save` when no cookiejar is specified, in
   case the caller explicitly sets it (which shouldn't happen since it
   wouldn't make sense, but whatever)

4. Clean up `open` logic resulting from the above three points