Use slack message attachments for color coding

This PR uses slack message attachments to introduce color coding of build status.

Previously when using hipchat, one thing I enjoyed was the color coding applied to messages.  'yellow' for started messages, 'green' for success, and 'red' for failure.  The current slack notify plugin does not provide such functionality.

Current:

![drone-slack-current](https://cloud.githubusercontent.com/assets/39340/4217895/a1375620-38ec-11e4-914a-6adda42d3266.png)

New:

![drone-slack-new](https://cloud.githubusercontent.com/assets/39340/4217810/c5a2e282-38eb-11e4-9a9d-5f18a26a80fe.png)