[Feature Request] Support neo4j constraints

GORM constraints should create constraints on the neo4j side.

Below you can find some documentation:
http://neo4j.com/docs/stable/query-constraints.html

And also you can find some documentation about new neo4j 2.3 Property Existence Constraints:
http://neo4j.com/blog/neo4j-2-3-enterprise-applications-at-scale/

Start by reading the linked Neo4j constraint documentation and locating GORM's constraint handling and Neo4j integration. Determine which GORM constraints and Neo4j versions are in scope, then verify that the requested constraints are created on the Neo4j side.

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