Playbook URL Support

This pull request adds the ability to specify a 'playbook' URL in the check JSON, which will optionally get displayed as a link on the dashboard event detail modal.  This allows anybody who gets paged or otherwise looks at an event in sensu to be able to follow the 'playbook' link to view the wiki/gist/documentation for how to fix that issue.

This is a key part of scaling operation work across a broader team.

This is flexible because you can have a different playbook URL per check, or you can re-use them if multiple checks have the same basic diagnosis/fix.

One thing to note is that the dash gets the event data from the API, but then loads the playbook URL out of the dashboard config (if the check is present).  This works if you drop the same sensu config on all servers, but not if the dashboard does not have access to the config.

I admit I'm not quite up-to-date on my dashboard install, and not sure if this will work for everyone, but I'm psyched to get this functionality up on our install (and hopefully get more engineers on PagerDuty sometime soon!).