Copy exercise submission to clipboard - addresses issue #1614

Addresses issue #1614, copy submission to clipboard.

* Adds a button to the submissions/show view with a clipboard icon. (See image below)

* Uses [zClip - jQuery plugin](http://www.steamdev.com/zclip/).

![screen shot 2014-06-27 at 1 11 08 am](https://cloud.githubusercontent.com/assets/3708050/3408163/dc9b3f80-fdc1-11e3-80b3-58ca9628a331.png)