Promote volume plugins, prep for persistent vols

Move pkg/kubelet/volume/... to pkg/volume/...
Some renames to make the soon-to-come persistent volumes work clearer.

pkg/volume/empty_dir/empty_dir_unsupported.go

@@ -0,0 +1,26 @@
+// +build !linux
+
+/*
+Copyright 2015 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package empty_dir
+
+// realMountDetector pretends to implement mediumer.
+type realMountDetector struct{}
+
+func (m *realMountDetector) GetMountMedium(path string) (storageMedium, bool, error) {
+	return mediumUnknown, false, nil
+}