Extend PeerBook with Reputation

[wanderer]

It would be nice to have an instance of PeerBook that also tracked the reputation of peers. It could methods such as updateReputation(id, int) and getReputation(id) and setReputationThreshold(int) which would set a minimal reputation, if a peer drops below this rep, then it would get evected from the peerBook

[sdtsui]

Thanks @wanderer - will see about grabbing this after #3.

Notes to self:

• for now, makes most sense to subtract reputation in _peerDiscovery: L87
• May want to use some sort of log scale for reputation.