utils: Add missing "B" for pretty_size()

* Add missing unit for megabytes, should print "MB".
console: Add support for displaying elapsed time

* Display the elapsed time in the the response row.
* Very handy for passive performance analysis.
console: Add support for displaying transfer rate

* Display the rate in the the response row.
* Very handy for passive performance analysis.
Merge pull request #159 from kmanna/elapsed_time

Transfer Rate and Elapsed Time Statistics