* Send zeros for any nonexistent scoreboard states

Make sure that we report any unused worker states as having zero workers in
that state - otherwise Prometheus will continue to use the previous value for a
while.

* Update expected metric counts in tests

We now return metrics for each potential worker state, so there are a lot more
of them.

Added cpuload metric.
Updated readme with changes made.

Implement standard release procedure for exporters + add version cli flag

Implement standard release procedure for prometheus exporters. Use CircleCI to build new binaries using promu and release to github, docker and quay.

Will be useful for the following commits, by allowing the standard release procedure for prometheus exporters.

Fix to make DockerImage run anywhere

Added Dockerfile, updated README

Use github.com/prometheus/common/log

github.com/prometheus/log is an old package, migrate to use "log" within
common.

Scoreboard provides a more detailed breakdown of worker status, export and
parse that too.