suppr de namespaces et ajout de .bfc

- pas de namespace sur les classes `.clear`, `.clearfix` et
`.visually-hidden`
- ajout d'une classe `.bfc` (identique à `.mod`) et qui crée un contexte
de formatage block

less/_01b-base.less

@@ -169,7 +169,7 @@ var {
 /* ----------------------------- */
 
 /* hidden but not for an assistive technology like a screen reader, Yahoo! method */
-.@{kna-namespace}visually-hidden {
+.visually-hidden {
   position: absolute !important;
   border: 0 !important;
   height: 1px !important;

less/_02-layout.less

@@ -3,19 +3,20 @@
 /* ----------------------------- */
 
 /* module, gains superpower "BFC" Block Formating Context */
-.@{kna-namespace}mod {
+.@{kna-namespace}mod,
+.@{kna-namespace}bfc {
   overflow: hidden;
 }
 
 /* blocks that needs to be placed under floats */
-.@{kna-namespace}clear,
+.clear,
 .@{kna-namespace}line,
 .@{kna-namespace}row {
   clear: both;
 }
 
 /* blocks that must contain floats */
-.@{kna-namespace}clearfix,
+.clearfix,
 .@{kna-namespace}line {
   &::after {
     content: "";

sass/_01b-base.scss

@@ -163,7 +163,7 @@ var {
 /* ----------------------------- */
 
 /* hidden but not for assistance tools, Yahoo! method */
-.#{$kna-namespace}visually-hidden {
+.visually-hidden {
   position: absolute !important;
   border: 0 !important;
   height: 1px !important;

sass/_02-layout.scss

@@ -3,19 +3,20 @@
 /* ----------------------------- */
 
 /* module, gains superpower "BFC" Block Formating Context */
-.#{$kna-namespace}mod {
+.#{$kna-namespace}mod,
+.#{$kna-namespace}bfc {
   overflow: hidden;
 }
 
 /* blocks that needs to be placed under floats */
-.#{$kna-namespace}clear,
+.clear,
 .#{$kna-namespace}line,
 .#{$kna-namespace}row {
   clear: both;
 }
 
 /* blocks that must contain floats */
-.#{$kna-namespace}clearfix,
+.clearfix,
 .#{$kna-namespace}line {
   &::after {
     content: "";