Added support for standard transforms

-webkit- stuff only works on webkit browsers.  I'm pretty sure that using the standard `transform:` here works in most modern browsers.

index.html

@@ -75,7 +75,7 @@
 						<p>That's it, time to go back up.</p>
 						<br>
 						<a href="#/2">
-							<img width="178" height="238" data-src="https://s3.amazonaws.com/hakim-static/reveal-js/arrow.png" alt="Up arrow" style="-webkit-transform: rotate(180deg);">
+							<img width="178" height="238" data-src="https://s3.amazonaws.com/hakim-static/reveal-js/arrow.png" alt="Up arrow" style="transform: rotate(180deg); -webkit-transform: rotate(180deg);">
 						</a>
 					</section>
 				</section>