Graphite: Enable configuring conversion units

This change set enables users of the Graphite exporter to specify time conversion units for rates and durations, like the original Coda Hale library does (http://metrics.codahale.com/manual/graphite/).

In order to not break the current `Graphite` function signature, I created a `GraphiteWithConfig` which takes a `GraphiteConfig` holding all the configuration values needed. `Graphite` was then refactored to use `GraphiteWithConfig`.
Documentation was added to all exports too.