Add artifact log command

Description

Changes proposed in this merge request:

• Add artifact subcommand group
• Add artifact log command

This merge request, when approved, will close issue/bug: #234 (closed)

CHANGELOG/Release Notes

• Initial WIP version
• Ported to new CASCache and ArtifactCache APIs.

buildstream/_artifactcache/artifactcache.py

@@ -547,6 +547,27 @@ class ArtifactCache():
 
         return self.cas.extract(ref, path)
 
+    # get_virtual_directory():
+    #
+    # Get the contents of the artifact as a buildstream.storage.Directory.
+    #
+    # The returned Directory may become invalid if remove or prune are called.
+    #
+    # Assumes artifact has previously been fetched or committed.
+    #
+    # Args:
+    #     element (Element): The Element to get the Directory of
+    #     key (str): The cache key to use
+    #
+    # Raises:
+    #     ArtifactError: If the artifact did not exist.
+    #
+    # Returns: virtual Directory
+    #
+    def get_virtual_directory(self, element, key):
+        ref = self.get_artifact_fullname(element, key)
+        return self.cas.get_virtual_directory(ref)
+
     # commit():
     #
     # Commit built artifact to cache.