Added missing grid make-column mixins and fixed some inconsistencies

make-column() was still been use to generate "medium size" columns (col-sm), while make-small-column was creating the default columns (col).
I also changed @grid-float-breakpoint to @screen-small, as it was used in make-row() and I think it makes more sense here than the other.

less/mixins.less

@@ -459,8 +459,25 @@
   }
 }
 
-// Generate the columns
+// Generate the default columns
 .make-column(@columns; @gutter: @grid-gutter-width) {
+  position: relative;
+  float: left;
+  // Prevent columns from collapsing when empty
+  min-height: 1px;
+  // Inner gutter via padding
+  padding-left:  (@gutter / 2);
+  padding-right: (@gutter / 2);
+  @max-width: (@screen-small - 1);
+
+  // Calculate width based on number of columns available
+  @media (max-width: @max-width) {
+    width: percentage((@columns / @grid-columns));
+  }
+}
+
+// Generate the small columns
+.make-small-column(@columns; @gutter: @grid-gutter-width) {
   position: relative;
   // Prevent columns from collapsing when empty
   min-height: 1px;
@@ -469,46 +486,62 @@
   padding-right: (@gutter / 2);
 
   // Calculate width based on number of columns available
-  @media (min-width: @grid-float-breakpoint) {
+  @media (min-width: @screen-small) {
     float: left;
     width: percentage((@columns / @grid-columns));
   }
 }
 
-// Generate the column offsets
-.make-column-offset(@columns) {
-  @media (min-width: @grid-float-breakpoint) {
+// Generate the small column offsets
+.make-small-column-offset(@columns) {
+  @media (min-width: @screen-small) {
     margin-left: percentage((@columns / @grid-columns));
   }
 }
-.make-column-push(@columns) {
-  @media (min-width: @grid-float-breakpoint) {
+.make-small-column-push(@columns) {
+  @media (min-width: @screen-small) {
     left: percentage((@columns / @grid-columns));
   }
 }
-.make-column-pull(@columns) {
-  @media (min-width: @grid-float-breakpoint) {
+.make-small-column-pull(@columns) {
+  @media (min-width: @screen-small) {
     right: percentage((@columns / @grid-columns));
   }
 }
 
-// Generate the small columns
-.make-small-column(@columns; @gutter: @grid-gutter-width) {
+// Generate the large columns
+.make-large-column(@columns; @gutter: @grid-gutter-width) {
   position: relative;
-  float: left;
   // Prevent columns from collapsing when empty
   min-height: 1px;
   // Inner gutter via padding
   padding-left:  (@gutter / 2);
   padding-right: (@gutter / 2);
-  @max-width: (@grid-float-breakpoint - 1);
 
   // Calculate width based on number of columns available
-  @media (max-width: @max-width) {
+  @media (min-width: @screen-medium) {
+    float: left;
     width: percentage((@columns / @grid-columns));
   }
 }
 
+// Generate the large column offsets
+.make-large-column-offset(@columns) {
+  @media (min-width: @screen-medium) {
+    margin-left: percentage((@columns / @grid-columns));
+  }
+}
+.make-large-column-push(@columns) {
+  @media (min-width: @screen-medium) {
+    left: percentage((@columns / @grid-columns));
+  }
+}
+.make-large-column-pull(@columns) {
+  @media (min-width: @screen-medium) {
+    right: percentage((@columns / @grid-columns));
+  }
+}
+
 
 // Form validation states
 //