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/storage/donut/interfaces.go

@@ -0,0 +1,50 @@
+package donut
+
+import (
+	"io"
+)
+
+// Collection of Donut specification interfaces
+
+// Donut interface
+type Donut interface {
+	CreateBucket(bucket string) error
+	GetObjectReader(bucket, object string) (io.ReadCloser, error)
+	GetObjectWriter(bucket, object string) (ObjectWriter, error)
+	GetObjectMetadata(bucket, object string) (map[string]string, error)
+	ListBuckets() ([]string, error)
+	ListObjects(bucket string) ([]string, error)
+}
+
+// Bucket interface
+type Bucket interface {
+	GetNodes() ([]string, error)
+}
+
+// Node interface
+type Node interface {
+	CreateBucket(bucket string) error
+	GetBuckets() ([]string, error)
+	GetDonutMetadata(bucket, object string) (map[string]string, error)
+	GetMetadata(bucket, object string) (map[string]string, error)
+	GetReader(bucket, object string) (io.ReadCloser, error)
+	GetWriter(bucket, object string) (Writer, error)
+	ListObjects(bucket string) ([]string, error)
+}
+
+// ObjectWriter interface
+type ObjectWriter interface {
+	Close() error
+	CloseWithError(error) error
+	GetMetadata() (map[string]string, error)
+	SetMetadata(map[string]string) error
+	Write([]byte) (int, error)
+}
+
+// Writer interface
+type Writer interface {
+	ObjectWriter
+
+	GetDonutMetadata() (map[string]string, error)
+	SetDonutMetadata(map[string]string) error
+}