Adds a new class called .dropdown-submenu-left. It is useful when you
have a .pull-right DropDown that has submenus. Without this, the menu
opens left past the page boundary, which can trigger the scrollbars and
cause other nastiness.

The problem was that the CSS selector made *all* of a submenu's nested
menus visible. Fixed by applying the selector to the immediate nested
menu only.

fix dropdown vars and make hover state be link color again, add gradient to dropdown link hovers, fix caret positions and active states on nav tabs, tweak nav link padding

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.

IE6 does not seem to be supported by Bootstrap, and reducing the number
of unused and invalid properties is always nice.

* 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