Download files from slack and reupload to mattermost (slack/mattermost). Closes #255

Refactor message.Extra to a map[string][]interface{} to have a bit more flexibility
for stuffing extra stuff.

For attached files from slack, files < 1MB size get downloaded (in memory), and get
put into Extra["file"][]config.FileInfo (containing a pointer to the buffer and
the filename). This is not async so slack channels with lots of attached files
may suffer a slowdown. (the download timeout is set at 5 seconds).

gateway/gateway.go

@@ -211,7 +211,7 @@ func (gw *Gateway) ignoreMessage(msg *config.Message) bool {
 	}
 	if msg.Text == "" {
 		// we have an attachment
-		if msg.Extra != nil {
+		if msg.Extra != nil && msg.Extra["attachments"] != nil {
 			return false
 		}
 		log.Debugf("ignoring empty message %#v from %s", msg, msg.Account)