fixes typo introduced in 2b6ec389

Problem introduced in 6b23a66c

accordion's data-parent can't contain dots

Conflicts:
	_layouts/customize.html

Update to use @list-group-border-radius

Added panel-variant mixin

apparently PRs against twbs/bootstrap have the same Travis slug

Fixed #9196 - malformed HTML in doc

Fixed link to JavaScript buttons plugin

Stray <h3> was being closed by an </h2>. Updated to valid HTML. Fixes #9196

This PR significantly improves Bootstrap's accessibility for users of assistive technology, such as screen readers. Some of the these changes add additional markup to the source examples, but we believe that the sacrifice in readability is worth achieving more widespread usage of accessibility best-practices.

What was done
- Added lots of [WAI-ARIA attributes](http://www.w3.org/WAI/intro/aria)
- Added `.sr-only` helper class, that is only readable by screen readers (and invisible for all other users). This lets us - make progress bars and paginations accessible to screen reading users.
- Advised users to always use label elements. For inline forms, they can hide them with `.sr-only`
- Added 'Skip navigation' link
- Added "Accessibility" section to getting-started.html.

What *wasn't* done
- Contrast issues (twbs#3572)
- Tooltips (twbs#8469)
- Documentation re: usage of icons, since they now live in a separate repo

Major props to all that contributed: @bensheldon...

fixes #8959: only negative indent rows within .container (prevents horizontal scroll on full-width pages)

disable BrowserStack for folks w/o their own key who are making PRs

fix scrollSpy instructions for JS call

Conflicts:
	less/mixins.less

Per @mdo's request.
Can re-enable IE after @fat figures out wassup w/ the unittest failures.

BS3: Add third party support docs

/cc @cv