Adds new comment screen

Adds new comment screen when click pitch in on the discussion page;
- When 'hide' is clicked, this screen slides to the bottom;

![untitled](https://cloud.githubusercontent.com/assets/4236592/5459579/13299a8c-8554-11e4-9092-e7513505e3e4.png)

Shouldn't the `cursor: pointer` be on the `.Button` class?