Package p2p (PoC7)

Here we go. That took a bit longer than expected.
The tests could be better, but we can improve them while integrating PoC 8 changes.

@zelig PTAL
This is still part of #155.
This won't work because message content is always an RLP list.
I'll change it to decode into `[1]DiscReason` tomorrow and add a test for it.