memleak fix

I hope you will like this one. 

Sometimes espetialli if I forget to turn off rtmpsnoop I experience huge memory consuption after some time. There was a quite huge memory leak because scapy by default save all incoming traffic in to memmory. 
Just try to run rtmpsnoop for a couple of hours during watching hulu or something and you will be suprised how much memmory it consumed.

This should set scapy to erase packets from memmory after you read them.