fix reconstructVXX/downloadVixFutures.py to do os.path.join() instead of + '\\' + which does not work on Unix

```
reconstructVXX/downloadVixFutures.py

has exception on Unix (OS X)

Please provide any additional information below:
use os.path.join() instead of + '\\' + which does not work on Unix
you can also use + '/' + but its not ideal


```

Original issue reported on code.google.com by `udi.benr...@gmail.com` on 17 Nov 2013 at 7:40
