Code restructuring, assigning proper subsystems to each

- Drivers contain
    * donut/*
    * file/*
    * memory/*

 - Storage format contains
    * donut/*
      - GetObject() --> renamed to GetObjectReader()
      - Deleted stale objectwriter.go, renamed donutwriter.go to object_writer.go

Simplifying, and documenting codebase further

pkg/drivers/bucket_policy_compat.go

@@ -0,0 +1,52 @@
+/*
+ * Minimalist Object Storage, (C) 2015 Minio, Inc.
+ *
+ * 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 drivers
+
+// This file implements compatability layer for AWS clients
+
+// Resource delimiter
+const (
+	AwsResource = "arn:aws:s3:::"
+)
+
+// TODO support canonical user
+// Principal delimiter
+const (
+	AwsPrincipal = "arn:aws:iam::"
+)
+
+// Action map
+var SupportedActionMapCompat = map[string]bool{
+	"*":                     true,
+	"s3:GetObject":          true,
+	"s3:ListBucket":         true,
+	"s3:PutObject":          true,
+	"s3:CreateBucket":       true,
+	"s3:GetBucketPolicy":    true,
+	"s3:DeleteBucketPolicy": true,
+	"s3:ListAllMyBuckets":   true,
+	"s3:PutBucketPolicy":    true,
+}
+
+func isValidActionS3(action []string) bool {
+	for _, a := range action {
+		if !SupportedActionMapCompat[a] {
+			return false
+		}
+	}
+	return true
+}