Remove dependency on external images

The "x" and the disclosure arrow are styled text/html elements. The search icon and the spinner are base64 encoded (only once each in select2.css). Makes it look sharper and simplifies "installation."
![screen shot 2013-10-08 at 3 08 16 pm](https://f.cloud.github.com/assets/3277153/1292504/82149a8c-3055-11e3-8236-92d002962636.png)