S3: Support signature=v4

To use S3 Signature Version 4. Some S3 services seem to require v4, while
others may only support v2, which remains the default.

I'm also not sure if v4 works correctly in all cases, there is this
upstream bug report: https://github.com/aristidb/aws/issues/262
I've only tested it against the default S3 endpoint.

Remote/S3.hs

@@ -99,6 +99,8 @@ remote = specialRemoteType $ RemoteType
 				(FieldDesc "port to connect to")
 			, optionalStringParser requeststyleField
 				(FieldDesc "for path-style requests, set to \"path\"")
+			, signatureVersionParser signatureField
+				(FieldDesc "S3 signature version")
 			, optionalStringParser mungekeysField HiddenField
 			, optionalStringParser AWS.s3credsField HiddenField
 			]
@@ -148,6 +150,22 @@ protocolField = Accepted "protocol"
 requeststyleField :: RemoteConfigField
 requeststyleField = Accepted "requeststyle"
 
+signatureField :: RemoteConfigField
+signatureField = Accepted "signature"
+
+newtype SignatureVersion = SignatureVersion Int
+
+signatureVersionParser :: RemoteConfigField -> FieldDesc -> RemoteConfigFieldParser
+signatureVersionParser f fd =
+	genParser go f defver fd
+		(Just (ValueDesc "v2 or v4"))
+  where
+	go "v2" = Just (SignatureVersion 2)
+	go "v4" = Just (SignatureVersion 4)
+	go _ = Nothing
+
+	defver = SignatureVersion 2
+
 portField :: RemoteConfigField
 portField = Accepted "port"
 
@@ -877,7 +895,10 @@ s3Configuration c = cfg
 		Nothing
 			| port == 443 -> AWS.HTTPS
 			| otherwise -> AWS.HTTP
-	cfg = S3.s3 proto endpoint False
+	cfg = case getRemoteConfigValue signatureField c of
+		Just (SignatureVersion 4) -> 
+			S3.s3v4 proto endpoint False S3.SignWithEffort
+		_ -> S3.s3 proto endpoint False
 
 data S3Info = S3Info
 	{ bucket :: S3.Bucket