OGM-547 Apply unique constraints on Neo4j database

  A unique constraint is going to be created for properties representing
  an identifier, a natural id or a unique column.

  Only unique constraints for single columns are created because Neo4j
  does not support constraint on mulitple properties.

  The creation of constraints for entities can be skipped setting the
  property hibernate.schema_update.unique_constraint_strategy to SKIP
OGM-547 Add test for unique constraints in Neo4j
OGM-547 Add test for skipping unique constraints generation
OGM-547 Update javadoc