implemented the sender worker for metrics
added test and comments after first round of reviews
second round of of review, support bulk calls in setting sent,
added more mocking in tests
support bulk calls in the sendmetrics api call
updates after merging with trunk
Changes from 3rd round of reviews
minor test changes after 4th round of reviews
Merge pull request #676 from mattyw/add-metric-sender-worker

Implemented the sender worker for metrics

Implemented a sender worker that will send metrics to a given service
Merge branch 'master' of https://github.com/juju/juju into agent-api-new-version-for-networker