Store tokens instead of token IDs for nodes.

The tcf.Graph API used to store token IDs. This reflects the XML
serialization, but did not match the rest of the API. It has been changed
to store references to the tokens.
Store tokens for graph edges and add 'unique' parameter.

This commit contains two related changes:

* Graph edges now track the tokens they represent. This is exported to TCF.
* The 'unique' parameter to `Graph.edge_for_tokens()` determines if an edge
  is only added once for two tokens.
Fix bug in TCF import.
Add worker name to debug output.
Fix typo in docstring.
Change Write worker to return its input unchanged.

This allows to use Write at any point in a chain to write out intermediate
results.