Add better support for dates by adding the dates in the abstract logic if needed
Add download of BouletCorp comics
Download Calvin and Hobbes
Move responsability to download images to the GenericComic.update() method

The get_next_comic method returns a dict with a 'url' attribute corresponding
to a (potentially empty) list of url of images. The update method will use it
to try to retrieve the images and set the 'local_img' attribute which will
corresponding to the images that were successfully downloaded.

In order to set the prefix to be use, a prefix attribute can be added but is
optional.

This makes things more consistent for comics with one image and comics with a
variable number of images.
Fix bug in date handling for download of C&Hobbes