Adding PartitionKeyIndexableGraph.

I would like to use both a PartitionGraph and a SailRepository. SailRepository requires a GraphSail which needs a KeyIndexableGraph. There is a PartitionIndexableGraph but no PartitionKeyIndexableGraph.

I have coded up a relatively simple PartitionKeyIndexableGraph (with test). However, I think there needs to be more careful handling around the createIndex. I'd be more than happy to complete this given some guidance.

Thanks.