Add Slack notfiications to Build and Deploy Plan Notification Recipients list on Cloud

Many organizations, like mine, use Slack (http://www.slack.com) for team-related communications. Slack has an awesome API for receiving POSTs. We should be able to add Slack as a notification recipient, possibly by allowing the user to configure endpoints under Bamboo Configuration (perhaps under the Communications tab). This way when builds hang or jobs succeed for the first time, whatever the organization requires, it can simply output that information in the Slack channel that the user configures.