Added support for a metrics collections

insert a batch of metrics instead of one at a time

only allow metrics to be added if the unit is alive
use right version of charm as per master
Merge pull request #466 from mattyw/add-metric-collection-to-state

Added support for a metrics collections

Added a metric collection for storing unit metric in the state server

Only support for:

* adding new metrics
* getting lists of unsent metrics
* setting the sent flag