DATAGRAPH-388 : First stab at getting Schema Indexes (Label based) in

@jexp - Request for feedback / comment on this initial stab at getting some of the Schema based Label functionality in. This PR looks to get the initial Indexed(indexType=Label) annotation functionality working including ensuring the schema based indexes are created as part of the MappingContextListener - although there are some major discussion points here, specifically the issue of needing to mix schema based commands (creating labels etc) alongside standard data updates, which cannot happen in the same transaction and this causes grief in various places. 

Once we agree on how to get around/proceed forward with some of these bigger issues, I shall tidy the code up. Let me know your thoughts!