Added support for using an <a> element with Button

Sometimes you wanna link out of your app, or to a separate page. So you use an `<a>` but still want to make it look like a "button".

Left `<a>`, right `<button>`
![button-with-a](https://f.cloud.github.com/assets/378023/762379/70a613f0-e7e6-11e2-92a8-f0c97b4a5bb5.png)

ps. There is something off with the number input on the iPad, but I'll wait to fix it since there are already a couple other PRs waiting.

Fixes #16