slave callbacks

I'm working on a slave device and thinking about porting your code to eCos and using it in my device. Your current implementation uses internal arrays for the register updates. In most embedded devices you would really like to have a callback than the user can implement to affect device state based on the register written. I would recommend adding slave callbacks that default to updating internal arrays but can be replaced with user define callback functions. I'll probably do this anyway so let me know if you have a preference for where the callbacks should live. I was thinking of just adding them to the mapping structure but it might be better to have a separate callback functions structure. I'll send you any changes I make if you'll let me know the best way to do it.