Added CreateClient, DeleteClient, CreateNode, DeleteNode functions

I have added these functions to simplify the creation of clients and nodes in Chef. 
I've started to use the go-gitlab client from Google and it's laid out pretty well. It provides different services on the client that you can use to reach the different types in the API. We could do something similar for the chef types. It seemed pretty good to me, I have forked it for a gitlab api client and i'm using the same pattern internally here to create more api clients.

http://godoc.org/github.com/google/go-github/github