calfw-cal: Represent Emacs diary-blocks as calfw periods

Please see https://github.com/kiwanami/emacs-calfw/issues/43 for more
details.
Bug fix: #35 Wrong merging for multiple entries in a day.

Thanks @cdeleuze!
Merge pull request #44 from neiljerram/master

calfw-cal: Represent Emacs diary-blocks as calfw periods