Ability to download files in CEF 3

```
As discussed: https://groups.google.com/forum/#!topic/cefpython/J5dJKuae-j4

DownloadHandler does not yet work with CEF3 (works with CEF1). Attempt to 
request a content-disposition with attachment results in an error currently.
```

Original issue reported on code.google.com by `sam...@gmail.com` on 18 Sep 2013 at 12:43
