DATAGRAPH-482 Allow to override property-name with @GraphProperty

Added "Property" annotation to explicitly set Node property names (something similar to @Column annotation in Hibernate).

Example:
@NodeEntity
public class Artist extends IdentifiableEntity {

	@Property(name = "first_name")
	private String firstName;

	@Property(name = "last_name")
	private String lastName;

    public Artist() {
    }

    public Artist(String firstName, String lastName) {

        this.firstName = firstName;
        this.lastName = lastName;
    }
}

Moved Property.name into GraphProperty.propertyName (according to the discussion in the pull request #188)
Added control to test that GraphProperty.propertyName is not null and not empty.
Added test to ensure GraphProperty works fine even with GraphProperty.propertyName == "".
See @GraphProperty annotation on the Artist's attributes 'born' and 'died'.