Add python3 example and update payload test code

[DevelopMan]

I found an issue with an example that uses this code:

python -c 'print "\x00"*20 + "\x80\xa0\x04\x08"'

It didn't work for me with pythons 3:

python -c 'print "\x00"*20 + "\x80\xa0\x04\x08"'

$ hexdump -C test_binary
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 c2 80 c2 a0  04 08 0a                 |...........|

As we can see we have some bytes that we didn't add.
The solution could be to use python2 or just use bash:

echo -n -e '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80\xa0\x04\x08' > ./just_do_it-56d11d5466611ad671ad47fba3d8bc5a5140046a2a28162eab9c82f98e352afa

I also updated the payload script, because for python3 it didn't work either.