Add support for split button dropdowns in justified button groups (with IE8 support)

Created by: MohammadYounes

The fixed values 26px/36px are calculated based on caret size, the only variable available for this is @caret-width-base/@caret-width-large.

caret size = .dropdown-toggle padding * 2 + .btn border * 2 + ( .caret border or .caret border lg) * 2

Sample: http://jsfiddle.net/seLk2khL

less/button-groups.less

@@ -217,6 +217,28 @@
     width: 100%;
   }
 
+  > .btn-group > .btn:first-child{
+    padding-right: (2 * @caret-width-base + 18);
+    margin-right: (-2 * @caret-width-base - 18);
+  }  
+  > .btn-group-lg > .btn-lg:first-child {
+    padding-right: (2 * @caret-width-large + 26);
+    margin-right: (-2 * @caret-width-large - 26);
+  }
+  //fallback to exclude non-split button dropdown
+  > .btn-group > .btn.dropdown-toggle:first-child {
+    padding-right: 0;
+    margin-right: 0;
+  }
+  > .btn-group > .btn + .dropdown-toggle {
+    z-index: 3;
+    float: right;
+    width: auto;
+  }  
+  > .btn-group:last-child > .btn:first-child {
+    .border-right-radius(@border-radius-large);
+  }
+  
   > .btn-group .dropdown-menu {
     left: auto;
   }