Overhaul semantic grid mixins

- Drops all the `.make-**-column` mixins for a single set to be used
within (or outside) media queries.
- Renames `.container-fixed` to `.make-container` for consistency.
- Updates built-in grid system to use new setup.

See http://jsbin.com/qiqet/2/ for an example of it in action.

less/grid.less

@@ -8,7 +8,7 @@
 // Set the container width, and override it for fixed navbars in media queries.
 
 .container {
-  .container-fixed();
+  .make-container();
 
   @media (min-width: @screen-sm-min) {
     width: @container-sm;
@@ -28,7 +28,7 @@
 // width for fluid, full width layouts.
 
 .container-fluid {
-  .container-fixed();
+  .make-container();
 }