Proxying downloaded files

Similarly to proxying video files Ggr is able to return any files downloaded by browser in running Selenium session.

Downloaded files are expected to be stored on the hub hosts and accessible via the following URL:
```
http://hub-host.example.com:4444/download/<real-session-id>/filename.txt
```
Such notation for example is supported by WebSummoner.
To get downloaded file via Ggr just use the same request but with the session ID returned to test:
Terminal window
```
curl http://ggr-host.example.com:4444/download/<test-session-id>/filename.txt
```