Draft of noise-libp2p spec (#202)

* first draft of noise-libp2p spec

* fix Alice & Bob ordering

* restructure & introduce Noise Socket for negotiation & framing

* specify Noise prologue data format

* clear up language around signatures in handshake

* Alice's message is included in prologue when switching protocols

* remove explicit negotiation & define libp2p API

* fix links

* fix return type of Secure{In|Out}bound methods