Add an optional http health endpoint

[arnecls]

Currently we (and probably others, too) use the metrics provided by gollum to do health checking.
A better way would be to allow plugins to return a health state that can be queried by a given http endpoint.

• Add a new commandline flag to gollum to open up an http endpoint like gollum --health=:80
• A general health check should be added to /health (maybe based on error/warning messages per second)
• Health checks should be added to /health/plugin_id
• Plugins can implement a HealthCheck interface GetHealth() (Status, string) where status can be of ok, warning or critical
• We should discuss wether to use different http status codes and if yes, which ones depending on the returned status

[ppar]

Fixed by PR #105 commit f1a0109