Add CSV output option

This PR has got a slight patch which adds --csv option which will cause result to be printed in CSV format (ping, download, upload).  I needed that so I can periodically do simple speed-tests from cron but thought it may be useful to others as well.

Sample output:
```
:~# speedtest --csv
7.312,496.76,106.82
```

I wonder if you could merge that with master and include in the next release?