Comment blocks support

Initial support for `@comment` bibtex entries. It seems to work, but I'm working on separating the parser from the actual generated object, otherwise things are starting the getting messy quickly.
Why do you want  to deprecate this function? It's true we can access the attribute variable, but I think this function is the symmetric of get_entry_dict() and from the user point of view, it's easier. What do you think?