Implemented gossip algorithm, described in detail in the gossip.go package comments.

Summary:
Split gossip into gossip manager (gossip.go), and server/client
components (server.go, client.go respectively). The gossip protocol
is outlined in messages.go. Finally, a utility for managing sets of
net.Addr objects is provided in addr_set.go.

The counting bloom filter ended up not being necessary, so I removed
that code. It's currently sitting in spencerkimball/cbfilter.

Generic exponential retry/backoff is in util/retry.go.

A simulation which outputs dot graphviz files is in simulation/gossip.go.

Test Plan: Unittest, though more to come

Reviewers: andybons

Subscribers: strangemonad

Differential Revision: http://phabricator.andybons.com/D3