Fix #1380 by switching to Clippy which is more "drop-in" and has better feedback

ZeroClipboard was trying to be too flexible by letting us style the button and such. As a result it was difficult to position the click target well.

Clippy (https://github.com/mojombo/clippy) is what GitHub uses (slightly modified in their new UI) and is an entirely self-contained button with user feedback:

![image](https://f.cloud.github.com/assets/7574/921497/30d02668-fefb-11e2-9d7a-807baf6c1800.png)

And on click:

![image](https://f.cloud.github.com/assets/7574/921499/4091548c-fefb-11e2-9b28-a83061df303f.png)

Fixes #1380