Add handler for invoking node on_connect/on_disconnect functions

The Discovery.NodeConnector.Handler behaviour can be implemented and passed
as an argument to Discovery.Poller.start_link/2. Everytime a node is connected to,
reconnected to, or disconnected from the functions each handler expose will be run.
Merge pull request #8 from undeadlabs/node-connector-handler

Add handler for invoking node on_connect/on_disconnect functions