PAP authentication added, do_build_payload changed to build_payload

[denysaleksandrov]

I added PAP Authentication and changed "do_build_payload" function to "build_payload" as the initail one was changed in scapy/packet.py.
I tested PAP, CHAP and both authentication value specified at the same time.
In order to add authentication I use ">>> p.authentication = ['chap', 'pap', 'both']":

p.authentication = ['chap']":

1. if only CHAP is specified and PPPoE server supports only CHAP or both CHAP and PAP
   then CHAP would be used

p.authentication = ['pap']":
2) if only PAP is specified and PPPoE server supports only PAP or both CHAP and PAP
then CHAP would be used

p.authentication = ['chap', 'pap']
or
p.authentication = ['both']
3) if PPPoE server sent CHAP challange, CHAP auth would be used
if PPPoE server didn't send anything, PAP auth would be used