Improve packngo Godoc

[t0mk]

At the moment, the godoc for Packngo doesn't read very well:
https://godoc.org/github.com/packethost/packngo

This would need improving the comments in the code, and add some summary, like in
https://github.com/gorilla/websocket/blob/master/doc.go

And then some examples
https://blog.golang.org/examples
https://github.com/gorilla/websocket/blob/master/example_test.go