Shortcodes: add the Recipes shortcode

See http://en.support.wordpress.com/recipes/
Fixes #605

Before the merge, I'd be happy to get a second set of eyes on `recipes/printthis.js`. It's a bit different from [the original](https://github.com/jasonday/printThis/blob/master/printThis.js) because I had to make some changes to pass JSHint tests. While most of them were harmless, @rase- had to jump in and help me with the IE hack on line 42 ([ref](https://github.com/jeherve/jetpack/blob/ec0e6266579e36dd14c2f9351c650517152f4a3b/modules/shortcodes/js/recipes-printthis.js#L42)).

The shortcode seems to work properly in all browsers in my tests.