Remove all #font mixins; replace single instance of its use in Bootstrap with the @font-family-monospace variable

* 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

* Uses .btn on label elements with nested checkbox and radio controls within
* Updated examples to reflect change in HTML and CSS
* Had to add .active all buttons for proper state highlighting (mimicing the :active pseudo state)
* Still needs JavaScript plugin updated by @fat

* Add new grid mixins adapted from Preboot (http://github.com/mdo/preboot)
* Drop the @grid-row-width and @grid-column-width variables
* Add the @grid-float-breakpoint variable, as used in Preboot
* Add support for .push* and .pull* classes for column ordering manipulation
* Document all the things

* Remove .darken-button-states and .buttonBackground mixins (latter wasn't being used anyway)
* Create new .btn-pseudo-states mixin for setting button background and border colors for default and pseudo states

* Instead of .divider-vertical, use .divider
* Makes use of .nav-divider mixin
* Dividers are horizontal to start, matching the default state of navbar nav links
* Dividers become vertical above 768px via media queries

horizontal three color gradient added

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).

this is a resubmission of #5875, but now against branch 3.0.0-wip
does not exclude elements under a fieldset's legend
issue #6058 is a duplicate of #5605

* Includes text inputs, selects, and textareas
* Updated docs to include .span* sizes wherever possible
* Commented out responsive 1200px inputs grid
* Still some derp to be done to improve more, but this is a decent first stab

Previously, we were using a generic label selector, but as this was changed in the main forms awhile back, we need to change it here.