Use <button> elements instead of <a href="javascript:void(0)"> in remove_button plugin

Using `<a href="javascript:void(0)">x</a>` to add an interactive button that will be triggered by JavaScript is a little bit 90s. The `<button>` element is a more appropriate click target in the remove_button plugin, and works identically. I have tweaked some of the styling so that the button doesn’t inherit any built-in browser styles.

I’ve been using this tweak in a project of mine since May and have tested in all major browsers back to IE9.

![screen shot 2014-12-28 at 20 27 36](https://cloud.githubusercontent.com/assets/4503044/5564887/6f55ada2-8ed0-11e4-87c8-8a36e19be18b.png)