New P2P protocol layer

[obscuren]

The new P2P layer should implement the (final) POC8 base protocol and should make it easy
to implement subprotocols such as whisper.

@zelig and @fjl are working on this.

Tasks:

• merge initial implementation (@zelig, PR: initial commit of p2p package #152)
• clean up protocol interface (@fjl, PR: P2P: Clean up protocol interface #180)
• separate Peer from Server (@fjl)
• fix the Server tests (@fjl)
• get up to date with PoC 7 (@fjl)
• implement eth protocol (@zelig)
• adapt the block pool (@zelig)
• replace the current peer layer (@zelig)

[zelig]

#230

[fjl]

This is now on develop.