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.
Fix responsive view for inverted navbar example
Fix link to navbar doc
Fix corss link for sticky footer
Replace png img by holder.js in carousel example
remove unused img from previous commit
Improve responsiveness for tablet and below of navbar in carousel example

- Remove margin-top of navbar-wrapper
- Remove border radius of navbar
- Space out the carousel
Merge pull request #7453 from burnsra/3.0.0-wip-nav

Fix responsive view for inverted navbar example
Fixes #7462: add radial-gradient spec to mixin
Merge branch 'example-fix' of https://github.com/Boulox/bootstrap into Boulox-example-fix
refactor some of the carousel caption stuff to center it and tighten styles up; update carousel template to start to redo mobile first
fix that sidenav
sidenote edit under customization section
Add small grid for phones and tablets
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
Added min-height to modal-header for modals with no header text
Fix typo in docs