to run it:
1) ./manage syncdb
2) add and parse a new corpus
3) test it:
http://localhost:8000/api/corpus/$corpus_id/graph?field1=journal&field2=ngrams
http://localhost:8000/api/corpus/$corpus_id/graph?field1=authors&field2=ngrams

Now, have to update the JSON format for the explorer.

[FEAT] Node_link function now with miam_id and stop_id and (to be tested) group_id. Need to be tested before merge.