Rename getNodeId() to getBNodeId()

Maybe I'm missing something, but I feel getNodeId() was probably meant to be named getBNodeId(), since it only caters for Bnodes, and not Nodes in general. 

See the other Bnode related functions:
```
newBNode()
  Create a new blank node in the graph and return it.
newBNodeId()
  Create a new unique blank node identifier and return it.
isBnode()
  Check to see if a resource is a blank node.
getNodeId()
  Get the identifier for a blank node
```

Also, isBnode() should probably be isBNode() if Bnodes are meant to have B and N uppercase. I don't care either way, I just think it would be nice to be consistent across the board.

Happy to roll a PR, just need to know if @njh would be open to such change ;)