Update Flowdock integration.

[xendk]

The Push API is deprecated: https://www.flowdock.com/api/push

The new Messages API allows for much nicer messages, but threatens with OAuth: https://www.flowdock.com/api/messages

Might as well fix #27 and part of #32.

[arnested]

Additional ideas:

• Add URL to site to message (customers can easier navigate to the site to view changes)
• Link to Github diff/compare (https://github.com/<user>/<repo>/compare/<sha>...<sha>)