refresh list of available classes if class is not found

This is an example of implementing a solution for #17.

I'm not too familiar with boost::function and boost::bind and not sure, if the below is best way to implement this. I basically just followed the existing style.

Please feel free to suggest better ways of implementing this - or add your own solution. :-)

PS: This is an important new feature addition, but does not change the API. Will it be possible to add this to Inidgo?