Refactor store.

* Use DataResolver and EntityResolver to resolve data_mappings and entity_mappings.
* Use EntityReference instead of Hash to manage entity_references.
Test EntityReferences.
Refactor Resolver::DataMapper, Resolver::EntityMapper and usage of both in Store.
Test Resolver module.
Test Resolver::DataMapper.
Test Resolver::EntityMapper.
Merge pull request #6 from holderbaum/store-use-entity-references-and-data-entity-resolver

Store use entity references and data entity resolver