Add support for icon emoji configuration within the Slack notifier

By default, incoming Slack webhooks are displayed with the generic webhook emoji:

![incoming_webhook_example1](https://cloud.githubusercontent.com/assets/123595/3912368/42d893dc-232a-11e4-86e8-a9fb81f6dddf.png)

This PR exposes a new `icon_emoji` attribute on the Slack notifier:

```ruby
notify_by Slack do |slack|
  slack.icon_emoji = ':ghost:'
end
```

![incoming_webhook_example3](https://cloud.githubusercontent.com/assets/123595/3912367/42d7e4f0-232a-11e4-8c87-d58eef248590.png)