chef-handler-datadog, v0.3.0.dev

This is a bit of a largish changeset, mostly due to adding testing into the mix to enable catching any regressions when refactoring.

Some review would be appreciated. I tried to set out the high-level details of the changes in [CHANGELOG.md](https://github.com/DataDog/chef-handler-datadog/blob/0.3.0/CHANGELOG.md#030--unreleased).

Comments are welcome. Target date for release is Wednesday, January 22 - at which time this branch will be merged, the version promoted to 0.3.0, and released to rubygems.