Add --bytes argument to get speed in MB/s

The :iphone: iPhone app has this feature quite nicely. I prefer to get 'real values' as it is not obvious what '25 Mbits/s' really means unless I decide to divide that by 8 as I read it.

While the iPhone app uses kB/s, I think it's better to wait till a better humanisation method is added for sizing before making the switch. So this just keeps it to MB/s.

I did not change the section to get the image for this because I do not know if the API can retrieve images like the one the iPhone app does. I suppose I could sniff it later to find out. :smirk_cat:

![iPhone app screenshot](http://www.speedtest.net/iphone/530979727.png)