New "template-parts" folder & Jetpack Infinite scroll

New file template organization + Jetpack Infinite Scroll support by @grappler.
This PR is referring to leopuleo #637, now closed, and discussion #619.

 
Sorry, I'm not sure what you mean...
The function to hook in `after_setup_theme` is `_s_jetpack_setup` and not `_s_infinite_scroll_render `.

I've pushed a commit where I've organized the functions and the hook as suggested in http://jetpack.me/support/infinite-scroll/
I hope it's fine. :smile: