Add .text-center utility class

I've personally found a very common use for this when developing with Ratchet.

sass/base.scss

@@ -81,6 +81,9 @@ a {
 .content-padded {
   margin: $bar-side-spacing;
 }
+.text-center {
+  text-align: center;
+}
 .pull-left {
   float: left;
 }