Added an icon_url.

#### What's this PR do?

Adds an icon to messages we post to Slack per [attachments docs](https://api.slack.com/docs/attachments) and [chat message docs](https://api.slack.com/methods/chat.postMessage).

#### Where should the reviewer start?

See the `default_data` dict we're now sending Slack.

#### How should this be manually tested?

n/a

#### Any background context you want to provide?

When we first integrated with Slack they didn't allow you to post an icon along with it. Now you can.

#### What are the relevant tickets?

n/a

#### Screenshots (if appropriate)

![2014-08-18 at 11 27 am](https://cloud.githubusercontent.com/assets/37667/3955827/517549c4-2705-11e4-9342-7d5f073e0b26.png)

#### What gif best describes this PR or how it makes you feel?

![](http://media2.giphy.com/media/8TRfrgKe1ndE4/giphy.gif)

#### Definition of Done:
- [ ] Is there appropriate test coverage? (e.g. ChefSpec, Mocha/Chai, Python, etc.)
- [ ] Does this PR require a Selenium test? (e.g. Browser-specific bugs or complicated UI bugs)
- [ ] Does this PR require a regression test? All fixes require a regression test.
- [ ] Does this PR require a blog post? If so, ensure marketing has signed off on content.
- [ ] Does the [knowledge base](https://sprint.ly/docs) need to be updated?
- [ ] Does this add new dependencies? If so, does Chef, M4, or PIP requirements need to be updated?
- [ ] Will this feature require a new piece of infrastructure be implemented?
- [ ] Are any new services fully automated in Chef?
- [ ] Are there ChefSpec tests for the new Chef recipes/cookbooks?
- [ ] Does this PR require new Mixpanel or GA events?
- [ ] Is there appropriate logging included?