Fixed navbar proposal attempting to fix #41

I've been watching some issues here in the browser-diet repo and one that particularly interested me was just closed. I'm referencing #41 here.

Since I saw you didn't do this yet due to the lack of time I decided to make my proposal.

It fixes the navbar at top without using jQuery when the users scrolls the page passing by the header.

In order to do so I had to modify some margins and add some paddings to make everything seamless.

I hope this don't break some rule of the project or something like that.

Also, I decided not to show the fixed bar on smartphones or small tablets. Actually anything below 768px wide.

What do you think?