remove tabindex=-1 as it was causing a weird select state in latest safari on yosemite when clicking anywhere in the negative space of the header

Add tabindex="-1" to main #content to work around bug in Chrome (and old
WebKits) and IE
Update advice in accessibility section
Include patch in docs CSS to suppress outline when #content is focused
(particularly as Chrome sets focus as result of mouse click for
tabindex="-1" elements)

Docs: Refactor to use .less file instead, importing Bootstrap vars and mixins, and cleaning all the things up

...at least if you're a writing nerd :P "Bootstrap ships with vanilla CSS, but *its* source code..."

X-Refs:
https://github.com/twbs/bootstrap/commit/b2a67ffe55f5fedb0ae1d5dff331687c3e744be0
https://github.com/twbs/bootstrap/commit/c97e6d5cad74d20257d28734f85311ef5d00b560

The Google Analytics account we use cannot currently be upgraded to Universal Analytics. In order to keep using Analytics, we have to stick with Classic for now.

Don't copy-pasta all the Sass docs because fuck keeping that up to date. Instead, point to https://github.com/twbs/bootstrap-sass/blob/master/README.md.