api: Implement multiple objects Delete api - fixes #956

This API takes input XML input in following form.

```
<?xml version="1.0" encoding="UTF-8"?>
<Delete>
    <Quiet>true</Quiet>
    <Object>
         <Key>Key</Key>
    </Object>
    <Object>
         <Key>Key</Key>
    </Object>
    ...
</Delete>
```

and responds the list of successful deletes, list of errors
for all the deleted objects.

```
<?xml version="1.0" encoding="UTF-8"?>
<DeleteResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Deleted>
    <Key>sample1.txt</Key>
  </Deleted>
  <Error>
    <Key>sample2.txt</Key>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
  </Error>
</DeleteResult>
```

api-errors.go

@@ -62,6 +62,7 @@ const (
 	InvalidCopyDest
 	MalformedXML
 	MissingContentLength
+	MissingContentMD5
 	MissingRequestBodyError
 	NoSuchBucket
 	NoSuchKey
@@ -125,7 +126,7 @@ var errorCodeResponse = map[int]APIError{
 	},
 	BadDigest: {
 		Code:           "BadDigest",
-		Description:    "The Content-MD5 you specified did not match what we received.",
+		Description:    "The Content-Md5 you specified did not match what we received.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
 	BucketAlreadyExists: {
@@ -165,7 +166,7 @@ var errorCodeResponse = map[int]APIError{
 	},
 	InvalidDigest: {
 		Code:           "InvalidDigest",
-		Description:    "The Content-MD5 you specified is not valid.",
+		Description:    "The Content-Md5 you specified is not valid.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
 	InvalidRange: {
@@ -183,6 +184,11 @@ var errorCodeResponse = map[int]APIError{
 		Description:    "You must provide the Content-Length HTTP header.",
 		HTTPStatusCode: http.StatusLengthRequired,
 	},
+	MissingContentMD5: {
+		Code:           "MissingContentMD5",
+		Description:    "Missing required header for this request: Content-Md5.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	MissingRequestBodyError: {
 		Code:           "MissingRequestBodyError",
 		Description:    "Request body is empty.",