Added support for showing info messages

[pennersr]

I think it would be a welcome addition for MTStatusBarOverlay to be able to show informational messages, next to progress/finished messages.

[myell0w]

Hey! Thank you for your contribution, looks good. I would like to merge this in, could you add the various forms of post(Immediate)InfoMessage s.t. it is similar to error/success messages? That means the different variants with various number of parameters that you have to specify. Thanks!