host_tracker: Add HostEvent event

We now raise an event when stuff happens with hosts
host_tracker: Fix exception when possible duplicate node
core: Register 'core' as a component

This allows you to listen_to_dependencies for core.
misc: Add gephi_topo component

Detects topology and streams it to Gephi, the great graph visualization
and manipulation application.
tk: Fix comment
ioworker: Add "notification" demo

This demonstrates some IOWorker usage both client and server side.
Also shows some Tk and adding py REPL commands.