Refactorings, docs and new metrics

Added

• All the metrics are now timestamped (#67)
• New ha_cluster_pacemaker_stonith_enabled metric (#68)
• Comprehensive metrics documentation
• RPM package spec and systemd unit are now in the repo rather than in the OBS package.

Changed

• Extensive refactoring to implement prometheus.Collector API (#66)
• Various metrics and their labels were renamed (#61, #62)
• Removed Go vendoring mode (#76)

Fixed

• Metrics no longer need to be reset (#60)
• TravisCI configuration (#74)

RPMs

https://build.opensuse.org/package/show/server:monitoring/prometheus-ha_cluster_exporter?rev=9