Unify fonts

This PR is for unifying font use on VirtKick webpage as mentioned in #8. The page was designed having *Open Sans* in mind and it seems that using this font alone seems to give the best effects, so my recommendation is to drop *Source Sans Pro* completely. I've left only Open Sans regular & bold and clean font sizes used on page.

@Nowaker: I would like to write some guidelines on how to use Open Sans on virtkick.io webpage, so we keep consistency in the future. I am wondering what place would be the best for it: source code, github wiki or maybe some other?