Allow public-key encryption of file content.

With the initremote parameters "encryption=pubkey keyid=788A3F4C".

/!\ Adding or removing a key has NO effect on files that have already
been copied to the remote. Hence using keyid+= and keyid-= with such
remotes should be used with care, and make little sense unless the point
is to replace a (sub-)key by another. /!\

Also, a test case has been added to ensure that the cipher and file
contents are encrypted as specified by the chosen encryption scheme.

Creds.hs

@@ -52,7 +52,7 @@ setRemoteCredPair c storage = go =<< getRemoteCredPair c storage
 		return c
 
 	storeconfig creds key (Just cipher) = do
-		s <- liftIO $ encrypt (GpgOpts []) cipher
+		s <- liftIO $ encrypt [] cipher
 			(feedBytes $ L.pack $ encodeCredPair creds)
 			(readBytes $ return . L.unpack)
 		return $ M.insert key (toB64 s) c