Make negative margins on rows the default, not just in containers, and add padding to body to compensate

fixes #9445: make img responsive mixin use display block, but provide option to pass custom value to it

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, @jasonlally, @criscristina, and @louh. Feel free to chime in, guys, if I've left anything out.

`img`s are no longer responsive by default. Instead, use
`.img-responsive` as a class or mixin. Existing `.thumbnail > img` and
`.img-thumbnail` classes now make use of this as a mixin as well.

IE8 doesn't need help with responsive images, they work just dandy apparently (see http://jsbin.com/idesit/1)

* Added a new mixin to calculate rems and pixels for IE fallback
* Updated the body, p, and .lead elements to use the new .font-size() mixin
* New mixin has default value of 14px or 1.4rem
* Still needs exploration, and change this means lots of work rejiggering margins and line-heights
* Related, started to reorganize the mixins a bit to call out mixins that are used by the framework and not utilities or snippets

* Same code, but different classes was confusing
* Now .img-thumbnail just extends .thumbnail

Mostly doubling-up :hover styles to also cover :focus, as a first step
to making the framework more keyboard-friendly.
Additionally, fixed two small markup issues in the docs/examples to
make the "Learn more" large primary button-styled links
keyboard-focusable (as without href they're treated as non-tabable
anchors).

* Port print styles from reset.less to scaffolding.less
* Port responsive image styles from reset.less to scaffolding.less

remove .border-radius mixin and replace with standard border-radius property, since -webkit-* and -moz-* are only needed for older versions