Dropdowns overhaul:

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.

less/navs.less

@@ -98,12 +98,10 @@
 .nav-tabs {
   border-bottom: 1px solid #ddd;
 }
-
 // Make the list-items overlay the bottom border
 .nav-tabs > li {
   margin-bottom: -1px;
 }
-
 // Actual tabs (as links)
 .nav-tabs > li > a {
   padding-top: 8px;
@@ -125,6 +123,7 @@
   cursor: default;
 }
 
+
 // PILLS
 // -----
 
@@ -189,14 +188,11 @@
 // DROPDOWNS
 // ---------
 
-// Position the menu
-.nav-tabs .dropdown-menu,
-.nav-pills .dropdown-menu {
-  margin-top: 1px;
-  border-width: 1px;
+.nav-tabs .dropdown-menu {
+  .border-radius(0 0 5px 5px); // remove the top rounded corners here since there is a hard edge above the menu
 }
 .nav-pills .dropdown-menu {
-  .border-radius(4px);
+  .border-radius(4px); // make rounded corners match the pills
 }
 
 // Default dropdown links