Fix transactional graph inheritance

[eparejatobes]

A better approach would be

1. create a transaction interface, with a reference to a transactional graph
2. transactional graph has methods for creating transactions etc

With this, in Neo4j we have Neo4jUntypedGraph which returns a wrapped Neo4j transaction while for titan we can make the graph itself implement both untyped graph and transaction

[eparejatobes]

Of course this can/should be mirrored at the TypedGraph level.

[eparejatobes]

Merging this