Implement EntityDereferenceEngine with an EntityDereferencer extension point

This will provide a core module define/providing

• EnhancementEngine implementation that dereferences all `fise:entity-reference`
• EntityDereference interface to be implemented by actual components that do support dereferencing of Entities
• Unit tests for the Engine implementation

Actual dereference engine implementation will

• implement the EntityDereference interface
• provide an OSGI Component taking the configuration, instantiating the provided DereferenceEngine implementation and register it as OSGI service.

The EntityDereference module will support

• OfflineMode (STANBOL-86)
• concurrent lookups of multiple Entities by using Java Concurrent ExecutorService.