Use Source Sans Pro from Typekit

This may be a bit controversial. This pull request changes all uses of Open Sans to Source Sans Pro. It also replaces all font loading (Google Fonts, Typekit) to use a single Typekit kit. Source Sans Pro's bold was a bit too heavy compared to Open Sans's bold, so I also replaced all instances of bold to semibold. Ideally we should also increase the font size by .1 because Source Sans Pro's x-height is a bit smaller than Open Sans', but I couldn't figure out how to do that without breaking the grid.

Discussion welcome.