Responsive layout

Hey! I just edited boilerplate to work in mobile for our project.

- Add $wrapper_width to variables
- Breakpoint for pad screens narrower than $wrapper_width
- Breakpoint for mobile screens narrower than $mobile_breakpoint
- Always embed Videos inside iframe and make video size responsive
(snipped borrowed from Bootstrap, see responsive_embed.css.scss)
- Switch to html5 and add viewport tag.
- Add support for css media queries/html5 for older IE

Please test esp. IE, I'm not sure what's your IE8 stresshold... Lemme know if something looks odd, thanks!