UI tweaks for better readability

I added a new layout, "pages", which is currently used in the first
page. I'm thinking in creating another one for the about/projects pages.

The biggest visual change is the increase in left/right-margins for the
central panel and a bit of "fine-tuning" for its padding.

Ah, I also divided _includes/navbar.html into social/links, so I could
add the social network list to my about page and remove the right menu
at the same time.