update(bootstrap): v3.1.1

Also redesigned some parts of the webpage.
* optimized for mobile
* new hero with black background
* navbar now white
* added shield horizontal line (copied from angulardart.org)
* new layout for code demos
* removed youtube videos for now
* changed fonts
* now using a vector graphic for the logo (svg)
remove pulsating code
style(cleanup) fix some linting errors
fix(css) more vendor prefixes
minor ui changes
fix(IE): try to support older browsers

add html5shim and modernizr. Detect browsers with
no svg support and show pngs instead.
less aggressive red
cleanup
re-add youtube links
feat(website): auto update download button

Update the version on the download button