Added bigram worker: returns a pickled bigram finder object
Added trigram worker as well
added tests for bigram and trigram workers
fixed bug, tests passing
Fix PEP8 + legibility in {bi,tri}gram workers
changed bigram and trigram workers to return tables in jSON format
Merge branch 'feature/bigrams' of github.com:NAMD/pypln.backend into feature/bigrams

Conflicts:
	pypln/backend/workers/bigrams.py
	pypln/backend/workers/trigrams.py
	tests/test_worker_bigrams.py
	tests/test_worker_trigrams.py
fix docstrings
Merge branch 'develop' into test_bigrams
Adds Bigrams to available workers
Adds regression tests for pickling of bigram worker output
Cast defaultdict to dict to avoid pickling the generator function
Fixes typo in test_mongo_store
Adds regression test to the error when saving worker output
Convert bigram worker output into a list of tuples so it can be saved
Merge branch 'feature/bigrams' into develop