golint fixes
correct side effects from lint corrections.
Align naming conventions with FreshBooks docs. Use 'Service URL' instead of 'org' name, as that is FBs nomenclature
Cleanup docs.
Increase test coverage, add support for caller to provide http.Client
Correct Invoice datatypes. Intruduce helper methods to allocate values and assign pointers to struct fields. Flesh out client.Do and related tests.