refactored metric sender into seperate package
Meter status made available to the uniter. The meter status is set on state and exposed to the charm
hooks in the form of environment variables. In addition, a new hook, meter-status-changed is executed
every time the meter status for that unit changes.
Merge pull request #817 from tasdomas/meter_status

Meter status

Meter status information available as hook context environment variables.

Meter status information will be set per-unit, once the metrics pipeline is in place.
fixes as suggested from reviews
fixed json tags for better readability
Merge pull request #866 from mattyw/refactor-metric-send

refactored metric sender into seperate package

Functions that dealt with sending metrics have been removed from state into a new apiserver/metricsender package.