Fixes #5594: Vertical button groups show use max-width, not width, and be scoped to immediate children buttons only

fixes #5177: remove color on .btn-large .caret, but keep sizing change; also start to add CSS tests for buttons

change up box-shadow mixin usage everywhere to use escaping again, or local var if necessary for further var evaluation

fixes #4718: add vertical-align: middle; to .btn-group to ensure proper alignment next to standard .btn

update button styles to use updated 14px base font size, align carets to others with no opacity changes

prevent button group buttons from wrapping in tight spaces by removing floats and replacing with font-size: 0; inline-block hack

remove @shadow instances and replace with escaped values, update mixins to not use a default value at all

fix other instances of mixins, swap use of box-shadow everywhere, include border-radius and transition

1. Removed broken and unused dropup examples from the navs/pills docs
2. New defaults for dropdown menus: all corners are rounded and always 1px offset from top unless otherwise specified
3. Refined active and open states for button dropdowns by adding a darker background color for each button variation when opened; also changed the opened dropdown-toggle's inset shadow to match that of an :active button
4. Generalized .dropdown.open to just .open, thus removing a few lines from button-groups.less.
5. Annnnnnnnnd I think that's it.

* Documents the hidden .dropdown-menu.bottom-up option (to be used for dropups)
* Extends the .bottom-up class to carets and all relevant caret styles
* Adds .dropdown-menu.pull-right class for right-aligned dropdowns and dropups
* Adds examples and some information to the "split dropdown" and "navbar" sections of the Docs