Improve line length, line height, font weight consistency

Came across the Conf Code of Conduct yesterday and I’m so glad this project exists. I wasn’t so glad about the line length.

So, this PR introduces a max line length of 660px, bumps up the line height to 25px (18/24 is just a little too tight for readability), and adds some font weight consistency to paragraphs (currently, everything is set at `font-weight: 200` – not only bad for readability on smaller font sizes, but Helvetica Neue on most machines does not include italics for this weight.)

Screenshot: https://www.dropbox.com/s/ndn7uxr9v5xsdp3/Screenshot%202014-02-25%2007.56.58.png