Add debug logging.

Could be helpful when troubleshooting issues with various providers if
the user is able to turn on debug logging. The most critical logging
provided is the request and responses sent and received from the oauth
servers.

src/storage.rs

@@ -227,7 +227,7 @@ impl<'de> Deserialize<'de> for JSONTokens {
         }
 
         // Instantiate our Visitor and ask the Deserializer to drive
-        // it over the input data, resulting in an instance of MyMap.
+        // it over the input data.
         deserializer.deserialize_seq(V)
     }
 }