added a CSS download button, fixes #1

I recreated the old plain download button

![old-download-button](https://f.cloud.github.com/assets/756669/304130/cba0962a-9635-11e2-9a28-40ce5e21c737.png)

in CSS and Unicode:

![new-download-button](https://f.cloud.github.com/assets/756669/304131/d0109b06-9635-11e2-8fba-84cff07d4111.png)

Don't know how to do browser detection without JavaScript.