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
Right, I'm convinced you need better checking both of unit names and charm urls. Neither of these are even valid.

(oh, and: I'm 99% sure a charm.URL serializes cleanly as a string -- let's just declare it as a charm.URL across the board)