Add button for toggling inline comments in diff view.

This is useful when there are many comments and they're becoming a distraction when trying to read the diff.

If it's not clear, the comments are toggled per file.

Comments visible:
![visible](https://f.cloud.github.com/assets/306980/1885823/73380d7e-79d8-11e3-8f77-3aef296f802c.png)

Comments hidden:
![hidden](https://f.cloud.github.com/assets/306980/1885825/787c4cf0-79d8-11e3-801b-7138b6407f78.png)