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.
Fixes #7213 Collapse plugin does not correctly set the "collapsed" class on accordion heading links
use font-size-base var
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
Added min-height to modal-header for modals with no header text
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
rename the navbar-inverse-brand variables for consistency
rename btn bg variables so that they are consistent with the other variables
fixes #7503: don't do dat
Merge pull request #7148 from retlehs/3.0.0-wip

Remove -webkit-backface-visibility on skew mixin
Merge remote-tracking branch 'upstream/3.0.0-wip' into 3.0.0-wip
hopefully fixed messed up stylesheet...
Merge pull request #7504 from acmetech/3.0.0-wip

variable update - fixed
better buttons on mobile
make back to top and big headings only for tablets and up