Add press partners section and pragmatic bookshelf logo

Hi,

I've just separated press partners from regular sponsors. I stayed consistent with the current layout of course. If you have another suggestion feel free to reject this pull request ;)

The pragmatic bookshelf logo doesn't have a transparent background and it didn't feel nice on the webpage, I added a little box-shadow, it's better but not perfect. Again, feel free to suggest anything.

Cheers,