ApiMappers::CaseMappper

In the effort of keeping responsibilities clear we split the json
serializing and the api key / value mapping.
The client serializes a case using CaseSerializer

This removes `Case#to_json`
Move document api mapping to ApiMappers
Move form field api mapping to ApiMappers
Move party api mapping to ApiMappers
ReferenceIdSerializer uses ReferenceIdMapper
Client uses ReferenceIdSerializer

This removes the need to `ReferenceID.to_json`.
`Client#get_case_url` can receive both a reference_id and a signature
case.
CaseMapper uses ReferenceIdMapper.to_api

We can now remove `ReferenceId#to_api_hash`
Merge pull request #3 from joeljunstrom/serializers-to-api-mappers

Serializers to api mappers