Utility class to load standard css files

It is possible to progrmmatically add/remove css files by using the `aria.utils.CSSLoader` singleton. Tags of type `<link>` are created and appended to the head of the document.

![cssLoader_coverage](https://f.cloud.github.com/assets/1152729/196475/8491cfca-801d-11e2-950f-e7777a49dbad.jpg)