`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

fix static navbar container issues, clean up docs presentation by unfixing the top navbar in favor of static one, start adding table of contents for longer sections of docs

fixes #3210: add .btn to .navbar-form to remove bottom margin, improve vertical align of contents of .navbar-form