Add span to .responsive-visibility and .responsive-invisibility mixins

[maktouch]

By doing that, I can make button labels disappear in small screen and shown correctly in big screen.

Example:

<button class="btn btn-primary"> <i class="icon icon-refresh"></i> <span class="hidden-xs"> Refresh </span> </button>

Code to modify:

.responsive-visibility() {
  display: block !important;
  table&  { display: table; }
  tr&     { display: table-row !important; }
  th&,
  td&     { display: table-cell !important; }
  span&   { display: inline !important; }
}

.responsive-invisibility() {
    &,
  tr&,
  th&,
  td&,
  span& { display: none !important; }
}

I've been using it for the last month - seems to work :).

[juthilo]

Duplicate of #12204, which features a more complete solution for all inline elements.