Support for wercker_deploy_url added

I've added support for the deploy url. Also changed is the way links are displayed for both builds and deploys. In slack you'll now get something like: 

   flenter/wercker-step-slack-notify: [deploy](http://wercker.com/) to production by Jacco Flenter failed.

No longer will you see the full url, just the word build/deploy as a link.