Create read model API.
Create IdentityService.
Correct exception $previous docs (may be null).
Adjust API service terminology.
Make API service return null when resource doesn't exist.
Rename to MalformedResponseException.

Rename MalformedReadModelResponseException to
MalformedResponseException.
Remove all references to ReadModel.
Correct IdentityService thrown exceptions and use statements.
Remove unused use statement.
Remove remaining references to "read models".
Accept all 2xx resource responses.
Merge pull request #5 from SURFnet/feature/read-model-api

Create read model API