replace clippy with zeroclipboard on gem pages

The copy/paste icons appeared to be a holdover from the previous design and didn't really mesh with the site post-redesign. It couldn't figure out if clippy supported changing the image so I reimplemented the icon using [ZeroClipboard](http://zeroclipboard.org/). ZeroClipboard is also better maintained so there's a plus there as well.

Here's what the icons look like now:

![image 2014-11-25 at 1 00 10 am](https://cloud.githubusercontent.com/assets/28044/5178668/7d4428d2-743e-11e4-8a02-405eb1bf0ca7.png)

The hover state is difficult to screenshot but the icons highlight in a reddish-orange that's consistent with the rest of the site.