Fixed affix-bottom positioning

Set top position to (scrollHeight - this.$element.height() - offsetBottom).
add role="tablist" to .nav-tabs in examples to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
add role="tab" to .nav-tabs [data-toggle="tab"]

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Merge pull request #13559 from twbs/carousel-role

add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility
Merge pull request #13558 from twbs/nav-tabs-role-tablist

add some ARIA roles to tabs for improved accessibility
Update html5shiv to 3.7.2.
Squashed commit of the following:

commit 81324491f4913d6797c3c9e4df83a1aefd7d9116
Author: Daniel <dano.husar@gmail.com>
Date:   Sat May 10 17:37:29 2014 +0100

    use opacity mixin

commit 694b0a3fcd196e8cba9e4c23850f903c0804cb44
Author: Daniel Husar <dano.husar@gmail.com>
Date:   Fri May 9 13:47:35 2014 +0100

    Update button-groups.less
Merge pull request #13541 from gpakosz/patch-2

Fixed affix-bottom positioning
Merge pull request #13549 from danielhusar/master

IE8 button group support