New env should default to in-cluster config

[johnSchnake]

When doing a simple API call example I had to do the following:

	c,err := envconf.NewWithKubeconfig("")
	if err !=nil{
		t.Fatalf("Failed to get in-cluster config: %v", err)
	}
	e := env.NewWithConfig(c)

I would have expected e := env.New() to use a default in-cluster config or something but I got a panic.

If I did something wrong let me know, but otherwise I think a tweak should be made to avoid those 4 extra lines of boilerplate, in-cluster-config setup.