Define Artifact diffs layers

It also add in simpledocker a naive implementation using the container-diff binary, which is supposed to go away with a proper API implementation.

pkg/compiler/artifact.go

@@ -15,11 +15,6 @@
 
 package compiler
 
-type Artifact interface {
-	GetPath() string
-	SetPath(string)
-}
-
 type PackageArtifact struct {
 	Path string
 }