Refactoring API handlers

- add responses for list-objects and list-buckets
- set `implement me` errors more useful

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/handler/response.go

@@ -0,0 +1,82 @@
+package handler
+
+import "encoding/xml"
+
+// ListBucketsResponse - format for list buckets response
+type ListBucketsResponse struct {
+	XMLName xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListAllMyBucketsResult" json:"-"`
+
+	Owner Owner
+
+	// Container for one or more buckets.
+	Buckets struct {
+		Buckets []Bucket `xml:"Bucket"`
+	} // Buckets are nested
+}
+
+// Bucket container for bucket metadata
+type Bucket struct {
+	Name         string
+	CreationDate string // time string of format "2006-01-02T15:04:05.000Z"
+}
+
+// Owner - bucket owner/principal
+type Owner struct {
+	ID          string
+	DisplayName string
+}
+
+// ListObjectsResponse - format for list objects response.
+type ListObjectsResponse struct {
+	XMLName xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult" json:"-"`
+
+	Name   string
+	Prefix string
+	Marker string
+
+	// When response is truncated (the IsTruncated element value in the response
+	// is true), you can use the key name in this field as marker in the subsequent
+	// request to get next set of objects. Server lists objects in alphabetical
+	// order Note: This element is returned only if you have delimiter request parameter
+	// specified. If response does not include the NextMaker and it is truncated,
+	// you can use the value of the last Key in the response as the marker in the
+	// subsequent request to get the next set of object keys.
+	NextMarker string `xml:"NextMarker,omitempty"`
+
+	MaxKeys   int
+	Delimiter string
+	// A flag that indicates whether or not ListObjects returned all of the results
+	// that satisfied the search criteria.
+	IsTruncated bool
+
+	Contents       []Object
+	CommonPrefixes []CommonPrefix
+
+	// Encoding type used to encode object keys in the response.
+	EncodingType string `xml:"EncodingType,omitempty"`
+}
+
+// CommonPrefix container for prefix response in ListObjectsResponse
+type CommonPrefix struct {
+	Prefix string
+}
+
+// Object container for object metadata
+type Object struct {
+	Key          string
+	LastModified string // time string of format "2006-01-02T15:04:05.000Z"
+	ETag         string
+	Size         int64
+
+	// Owner of the object.
+	Owner Owner
+
+	// The class of storage used to store the object.
+	StorageClass string
+
+	// UserMetadata user-defined metadata
+	UserMetadata StringMap `xml:"UserMetadata,omitempty"`
+}
+
+// StringMap is a map[string]string.
+type StringMap map[string]string