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.
Added min-height to modal-header for modals with no header text
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
don't use table for label variatons
cololol

Correct all span, push, pull, offset classes to col- schema
Fix a typo
Merge pull request #7523 from juthilo/3.0.0-wip

Fully update docs to col-...-* schema
Reordering variables that were being used before they were declared. Under some compilers (dotless & sublime 2 package), this would cause the compile to break. Variables in question were:

@dropdown-link-active-bg
@navbar-link-color
@navbar-inverse-link-color
Dusting the docs

* Fix some typos, grammar, sentence structure
* Fix left-over error tr class
* Remove uneditable inputs section
* Make compatibility sections stand out as sidenotes
* Make modal example scrollable on big screens, too
* Make some examples more self-explanatory