Make backends pluggable

[sarguru]

Currently the codebase seems to be more sensu based (since its intended to be an uchiwa replacement), but it would be cool to add an ability to plug in multiple backends (eg: pagerduty, riemann, circonus, datadog, nagios etc etc) and define templates for them so it can become a more general purpose visualiser than it is at the moment.

[mikepea]

PagerDoody added. It's still very much dependent on 'things that have a REST API that returns a list of events in JSON', but it's a start. And having PD integration is super useful.

[sarguru]

awesome 👍