recoco: Change API for Recv()
recoco: Change API for Recv()

Now returns None on error instead of empty bytes.  Not sure if this is really
a good change or not yet.

diogommartins/pox,adusia/pox,pthien92/sdn,denovogroup/pox,jacobq/csci5221-viro-project,VamsikrishnaNallabothu/pox,PrincetonUniversity/pox,kpengboy/pox-exercise,denovogroup/pox,diogommartins/pox,kulawczukmarcin/mypox,kpengboy/pox-exercise,denovogroup/pox,noxrepo/pox,jacobq/csci5221-viro-project,kpengboy/pox-exercise,xAKLx/pox,chenyuntc/pox,waltznetworks/pox,xAKLx/pox,kavitshah8/SDNDeveloper,denovogroup/pox,waltznetworks/pox,waltznetworks/pox,adusia/pox,adusia/pox,kavitshah8/SDNDeveloper,pthien92/sdn,denovogroup/pox,carlye566/IoT-POX,carlye566/IoT-POX,PrincetonUniversity/pox,pthien92/sdn,pthien92/sdn,adusia/pox,kavitshah8/SDNDeveloper,chenyuntc/pox,adusia/pox,waltznetworks/pox,PrincetonUniversity/pox,PrincetonUniversity/pox,andiwundsam/_of_normalize,noxrepo/pox,waltznetworks/pox,MurphyMc/pox,kavitshah8/SDNDeveloper,VamsikrishnaNallabothu/pox,kulawczukmarcin/mypox,kulawczukmarcin/mypox,VamsikrishnaNallabothu/pox,noxrepo/pox,MurphyMc/pox,kulawczukmarcin/mypox,xAKLx/pox,xAKLx/pox,PrincetonUniversity/pox,MurphyMc/pox,andiwundsam/_of_normalize,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,pthien92/sdn,noxrepo/pox,jacobq/csci5221-viro-project,VamsikrishnaNallabothu/pox,kpengboy/pox-exercise,MurphyMc/pox,diogommartins/pox,MurphyMc/pox,carlye566/IoT-POX,VamsikrishnaNallabothu/pox,xAKLx/pox,carlye566/IoT-POX,chenyuntc/pox,andiwundsam/_of_normalize,andiwundsam/_of_normalize,chenyuntc/pox,diogommartins/pox,kpengboy/pox-exercise,carlye566/IoT-POX,diogommartins/pox,chenyuntc/pox,jacobq/csci5221-viro-project