improve parsing of stepbytes (increment size) argument

[StefanoSalsano]

fixes issue #225 stepbytes (increment size) argument does not support 1M notation

if you run this command .build/all_reduce_perf -b 64M -e 70M -i 2M -g 1

the increment is not parsed as 2M but as 2 bytes, which is quite unexpected

[AddyLaddy]

Thanks!