Add support for side-by-side inline comments

- Only display inline comments for changed lines
- Support reply to existing inline comments
- **DO NOT** display inline comment for matched lines
  - Because we do not have line_code for matched lines in side-by-side mode
  - And I am not sure where should i put this comment, on the left or on the right?
  - We could support this by modifying in https://github.com/gitlabhq/gitlabhq/blob/master/app/helpers/commits_helper.rb#L108
- **DO NOT** support adding new inline comments
  - First, we also need line_code for matched lines
  - Second, it may involve more code changes. I could do it, but maybe in another pull-request.

Inline Mode
![screen shot 2014-04-24 at 8 55 49 pm](https://cloud.githubusercontent.com/assets/1321488/2789578/ccf50a50-cbaf-11e3-8be0-7035e9ebc913.png)


Side-by-side Mode
![screen shot 2014-04-24 at 8 49 00 pm](https://cloud.githubusercontent.com/assets/1321488/2789553/a12ae49e-cbaf-11e3-9e82-437b159b1294.png)