@orta => use a dictionary that subclasses can optionally extend to determine what nib to use

@orta I'm not sure if you actually have permission to merge this (into my own master branch) but you should still be able to comment, I think.

Previously there was a different method for each cell type that would return the appropriate nib. Now there is a dictionary using the kind of arbitrary names from the method names as keys (they're not exactly always the cell class name, maybe this can be addressed later), and the name of the appropriate nib as values.

This dictionary can optionally be merged with a dictionary provided in any subclass to add to it or change what nib will be used for exiting cell types.

Maybe these things should be class methods and not instance methods? Anyway give me your comments.