Adding entities with blank ids

Currently code like below

``` java
var entity = EntityContext.Create<IPerson>(new EntityId("urn:t:p"));
var friend = EntityContext.Create<IPerson>(entity.CreateBlankId());
entity.Friend = friend;
friend.FirstName = "D";
EntityContext.Commit();
```

Would create invalid data, because each entity would be inserted in a separate command, which created two distinct blank nodes.