support collectd threshold states in dashboard CSS

After this patch, pointing a tuned collectd install including [thresholds](https://github.com/collectd/collectd/blob/master/src/collectd-threshold.pod) will highlight threshold warnings and failures directly in the riemann dashboard.

Note this is not a "clean" change, as it includes adjustments to allow clean integration with collectd:

- Riemann uses ok|warning|critical to colour states in the dashboard.
- CollectD sends okay|warning|failure notifications via @pyr's `write_riemann` plugin, along with the original event.

Alternatively this could be patched on collectd side, or rewritten in `riemann.config`.