Add ability to copy Mixfile entry to clipboard

This PR adds an input on the opposite side of the package title/version header containing an the latest `{package, version}` entry to go into the Mixfile. When the box is clicked, the contents is hilighted to be easily copied.

This behavior is seen on [RubyGems](http://rubygems.org/gems/rails) under the __Gemfile__ section.

:pineapple: :banana: 

![screen shot 2014-10-04 at 4 15 11 pm](https://cloud.githubusercontent.com/assets/860934/4514787/d4c15c44-4b8d-11e4-92d3-f5ced884fcfa.png)
![screen shot 2014-10-04 at 4 15 24 pm](https://cloud.githubusercontent.com/assets/860934/4514788/d9d4b000-4b8d-11e4-87e8-2f9e149cb38e.png)