Fix warnings in PeerAddr.hs
Fix bugs in PeerAddr encoding.

Also:

* PeerAddr.hs internals and export list have been simplified;
* tests added.
Rename PeerAddr(peerAddr) to PeerAddr(peerHost)
Strictify peerHost field
Test peer list encoding
Add spec for AnnounceInfo encoding
Add BEP7 to BEPs list
Add TODOs and cosmetic fixes