Add expect initalization method

[volfco]

There should be an expect method added to ZkStruct that blocks until data appears in the assumed location. Like this:

zkstate::ZkState::expect(
    zk.clone(), 
    "/testing"
).unwrap()

Expect should block until /testing/payload is populated by someone else. This is useful when you're creating a ZkState object from a ZooKeeper Watcher firing, and you get a notification that a node is appearing and you want to create a local copy of it. Using new requires a bogus struct that might be visible to listeners locally before this library populates it.