add a progress callback to storeKey, and threaded it all the way through

Transfer info files are updated when the callback is called, updating
the number of bytes transferred.

Left unused p variables at every place the callback should be used.
Which is rather a lot..

Types/Remote.hs

@@ -36,6 +36,10 @@ instance Eq (RemoteTypeA a) where
 {- A filename associated with a Key, for display to user. -}
 type AssociatedFile = Maybe FilePath
 
+{- An action that can be run repeatedly, feeding it the number of
+ - bytes sent or retreived so far. -}
+type ProgressCallback = (Integer -> IO ())
+
 {- An individual remote. -}
 data RemoteA a = Remote {
 	-- each Remote has a unique uuid
@@ -45,7 +49,7 @@ data RemoteA a = Remote {
 	-- Remotes have a use cost; higher is more expensive
 	cost :: Int,
 	-- Transfers a key to the remote.
-	storeKey :: Key -> AssociatedFile -> a Bool,
+	storeKey :: Key -> AssociatedFile -> ProgressCallback -> a Bool,
 	-- retrieves a key's contents to a file
 	retrieveKeyFile :: Key -> AssociatedFile -> FilePath -> a Bool,
 	-- retrieves a key's contents to a tmp file, if it can be done cheaply