Quiero que las columnas Contenido A, Contenido B y Contenido C estén centradas horizontalmente. Tengo este código intentando agregar

http://jsfiddle.net/hsX5q/24/

HTML: margen: 0 auto a .columns-container y no funciona. ¿Alguien podría ayudar?

/*************************
 * Sticky footer hack
 * Source: http://pixelsvsbytes.com/blog/2011/09/sticky-css-footers-the-flexible-way/
 ************************/

/* Stretching all container's parents to full height */

html,
body {
  height: 100%;
  margin: 0;
  padding: 0;
}
/* Setting the container to be a table with maximum width and height */

#container {
  display: table;
  height: 100%;
  width: 100%;
}
/* All sections (container's children) should be table rows with minimal height */

.section {
  display: table-row;
  height: 1px;
}
/* The last-but-one section should be stretched to automatic height */

.section.expand {
  height: auto;
}
/*************************
 * Full height columns
 ************************/

/* We need one extra container, setting it to full width */

.columns-container {
  display: table-cell;
  height: 100%;
  width: 300px;
  margin: 0 auto;
}
/* Creating columns */

.column {
  /* The float:left won't work for Chrome for some reason, so inline-block */
  display: inline-block;
  /* for this to work, the .column elements should have NO SPACE BETWEEN THEM */
  vertical-align: top;
  height: 100%;
  width: 100px;
}
/****************************************************************
 * Just some coloring so that we're able to see height of columns
 ****************************************************************/

header {
  background-color: yellow;
}
#a {
  background-color: pink;
}
#b {
  background-color: lightgreen;
}
#c {
  background-color: lightblue;
}
footer {
  background-color: purple;
}

<div id="container">
  <header class="section">
    foo
  </header>

  <div class="section expand">
    <div class="columns-container">
      <div class="column" id="a">
        <p>Contents A</p>
      </div>
      <div class="column" id="b">
        <p>Contents B</p>
      </div>
      <div class="column" id="c">
        <p>Contents C</p>
      </div>
    </div>
  </div>

  <footer class="section">
    bar
  </footer>
</div>