stepbytes (increment size) argument does not support 1M notation

[StefanoSalsano]

the documentation of the stepbytes argument says:
-i,--stepbytes fixed increment between sizes. Default : 1M

actually, 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

I will open a pull request to fix this annoying behavior

[StefanoSalsano]

I have submitted pull request #226 to fix the problem