Fetch Messages with 'urgent' option.

This adds an optional parameter to fetch messages with an 'urgent' option. The old method without the option still works. I find this useful if I need to prioritise the fetching of messages, especially when there are a lot of flagging/moving operations being executed. 

I had to add the 'urgent' option to folderStatusOperation and folderInfoOperation as well because these are often used before messages are fetched, so wouldn't want those to be stuck waiting in queue.