Integration test overhaul

As well as adding more test coverage and some missing features, this set of changes detaches the integration tests from the live Esendex REST API.

Although quite sensible to integration test against the real REST API, it was proving difficult to secure the credentials required during the build process. To aid in the aspirations to make a sandbox for Esendex client libraries, this work facilitated the creation of one such sandbox which uses a RAML (see [raml-org/raml-spec](https://github.com/raml-org/raml-spec)) definition of the Esendex REST API and presents a server to test against using an [Osprey](https://github.com/mulesoft/osprey) instance.

Refer to the [esendex-sandbox](https://github.com/codesleuth/esendex-sandbox) project for details on the future of the project and its support. It is by no means comprehensive, and may be dropped.