restructure

lib/encoder merged into package enet (go mailing list insists you either make a separate repo for sub-libs or merge them into the sole package lib, when making a package)

enet/*.go moved to top level directory (so that it becomes "go get github.com/haltingstate/enet" instead of "go get github.com/haltingstate/enet/enet" which is unclear

main.go moved to main/ since enet/*.go is now in the repo root.  main.go is only useful to the developers for manual test inspection