Users can comment on pipeline run history.

Each pipeline run can have a single comment written next to it. This can be used to share failure information among teams looking at the pipelines without needing to review the logs.

Viewing comments, note the different button text:
![screen shot 2014-11-06 at 5 23 11 pm](https://cloud.githubusercontent.com/assets/24275/4947033/fff7802e-661c-11e4-9476-e49030c815ec.png)

Editing a comment:
![screen shot 2014-11-06 at 5 23 42 pm](https://cloud.githubusercontent.com/assets/24275/4947034/02615718-661d-11e4-8301-b0643a727248.png)
I think we sanitize `params[:comment]` before saving into DB rather than doing HTML escape on UI. what do you think?