Support Slack as a reporter

I added a slack client to report the deployment status to a Slack channel.

Following is a current sample configuration.

    messenger:
        type: slack
        channel: general
        url: https://hooks.slack.com/services/XXXXXXX
        username: takahi-i


@takahi-i  I think it is better `Post` function has `Post(h httpClient, message string)`, because it makes possible to **test** `Post ` method without send message to slack actually with mock httpClient object. But like other implementations in this trunk (`hipchat` etc..) don't have `httpClient` as arguments,  so It is impossible to change them at all, please solve this issue in another PR later,