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.
I think in this case the getter function is unnecessary, as it simply returns the attribute. `data.entry` and `data.entry_dict` are very simple and very pythonic I think. 

In my next pull request (sorry for the size!) I have actually made `entry_dict` a property which will use the relevant getter function (which is needed there so you only once calculate the dict).

I wouldn't suggest to remove the `get_...` methods for the forseeable future, though.