Make PeerGroup an Array subclass

- this allows us to do all the nice collections stuff without having to implement it

Signed-off-by: Scott Wilson <i@scottwilson.ca>
Rename and adjust AddrBook methods

- asPeerGroup -> peers (replaces old peers method)
- asOthersPeerGroup -> others
- onlinePeers returns a PeerGroup not an Array

Signed-off-by: Scott Wilson <i@scottwilson.ca>
ServerRegistry allows for other configs than 1 server per Peer

Signed-off-by: Scott Wilson <i@scottwilson.ca>