yui3 issue #1300 - images should not be used to draw triangles. we have css!

Hi @allenrabinovich,

I recently discovered a problem with problem with my project that uses yui when it was unable to locate an image file, causing the calendar nav controls to go missing. Upon inspecting the elements, I was a little surprised to see that they were not simply drawn with css. 

I decided to fix the problem on our end, but also send you the pull request as this will be an improvement for yui3 core.

before (triangle images)
![screen shot 2013-10-10 at 11 04 43 am](https://f.cloud.github.com/assets/1154834/1308928/88725fcc-31d6-11e3-9aec-c75f70bbc455.jpg)


after (css)
![screen shot 2013-10-10 at 10 59 35 am](https://f.cloud.github.com/assets/1154834/1308931/8d5be210-31d6-11e3-8598-a7777fe1358c.jpg)


http://css-tricks.com/snippets/css/css-triangle/

![dan_medium](https://f.cloud.github.com/assets/1154834/1308960/47276a02-31d7-11e3-8ccd-4d70118ced59.gif)