Adjust font-weight property of .lead for better readability on mobile devices

[See a screenshot of this change in action.](http://i.imgur.com/8fGD3Rv.png)

When on a mobile device (iPhone in this case), the **.lead** body copy class is displayed in a very thin font, which is overpowered by regular text. Kind of defeats the purpose, huh?

I've bumped up the **font-weight** in order to increase readability on mobile devices. It also wouldn't be a bad idea to bump up the font-size from 16px to 18px.