Added mixin for bordered arrows

Created by: scottkf

So you can get borders that look like this:

http://i.minus.com/ibmuYBGrlkwrTP.png

By using the mixin this way:

.arrow {
  #borderArrow > .below(15px, 1px, @color, #f9f9f9);
}

You can use .below, .above, .left, .right and the parameters are: width of the arrows, border width, color of the border, color of the inside

Given this HTML:

<div class="arrow">
  <div class="inner"></div>
</div>