Optional support for carbon-aggregator

Allow carbon-aggregator[0] to be configured and enabled. This is used by
GOV.UK to calculate deploys and HTTP status code rates. Class docs should be
fairly self explanatory.

[0]: https://graphite.readthedocs.org/en/0.9.12/carbon-daemons.html#carbon-aggregator-py

--

Also refactored tests and fixed a documentation typo.

/cc @rjw1 @KushalP @philandstuff