DATAGRAPH-500 - Add support for custom implementations in CDI repositories.

Original pull request: #198.
DATAGRAPH-500 - Polishing of the CDI extension.

Removed the default constructor in DelegatingGraphDatabase as its presences causes the type to become a managed bean. As the integration tests create a dedicated instance of GraphDatabase via a @Producer method we now fail to resolve a unique instance.

Made all repositories eager CDI beans as the lazy instantiation might break the transaction during query lookups.

Original pull request: #198.
DATAGRAPH-503 - Polished reference documentation.

Combined the forewords into the preface document.