Remove -webkit-backface-visibility on skew mixin

Ref #5319 - This rule is causing pixelated edges on iOS. Removing
it makes everything smooth across all OS's, browsers, and devices.
updated docs, changed out span* for col-span-*
Merge pull request #7469 from apotropaic/3.0.0-wip

updated docs, changed out span* for col-span-*
use font-size-base var
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
Fix typo in docs
Merge pull request #7495 from philfreo/3.0.0-wip

Fix typo in docs
Update docs.html

Added heads up regarding lone icons providing accessibility issues for screen readers.
fixes #7497: use var for jumbotron border radius
Merge pull request #7500 from JamieKnight/patch-3

Update docs to add warning about accessibility implications of using icons without any text
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
fixes #7503: don't do dat
Merge pull request #7148 from retlehs/3.0.0-wip

Remove -webkit-backface-visibility on skew mixin
Merge pull request #6 from twitter/3.0.0-wip

3.0.0 wip