enhancement: epub export

[andrearicci]

I would like to be able to export my recipes to epub and mobi formats, to take them with me when I'm around, or to give them to friends,
with the options to export:
*all recipes
*selected recipes
and being able to build the book making chapters based on different fields:
*by kind (entrees/main dishes/dessert…)
*by name
*by ingredients
*maybe by any field and also being able to organize the order in any arbitrary way.
Thank you

[ockham]

Duplicate of #57.