Corner support for stacked icons

I built out a little CSS to support the fa-stack-corner-* class for stacked icons, like this: 

<span class="fa-stack fa-large">
      <i class="fa fa-user fa-stack-1x"></i>
      <i class="fa fa-plus fa-stack-corner-se"></i>
</span>

You can specify an icon to be in the nw, ne, sw or se corner. The corner icons will size dynamically if placed next to a fa-stack-2x icon, or if given the class fa-inverse or if placed next to another icon with the classname fa-inverse.

I also added support for black and white text-shadows based on whether the corner icon or it's sibling is normal color or inversed.