Add unit tests

[alexzautke]

Add an unit testing framework to this project.

Initial requirements for unit tests:

• $document should return HTTP 200 with a valid FHIR document in the body (GET & POST)
• $document should return HTTP 404 when being called on a missing composition
• $document should persist the generated document if the persists parameter is set to true
• $document should return an INVALID_REQUEST when being called with POST and a missing id
• $document should return HTTP 500 if a local reference to a resource, which should be included in the document, can't be found.
• $document should return HTTP 500 if an absolute / external reference to a resource is requested to be included in the document.

[alexzautke]

Fixed by PR #9