ENH: Revamp mmm fixture import code

- Old fuzzy matching code was throwing away ~75% of the matches
  included in the file. expunged.
- Now relies on entity_id field already included in the fixture,
  providing 6x more matches, relying on a faster and more accurate
  algo.
- Closes the long-ignored #3303.
- Uses a single json fixture file.
- Added retrieval_date and schema_version to fixture file.
- Data lives at:
	https://github.com/y-p/mmm_scrape/blob/master/results/mmm.json
  ocasionally updated.
- moved test fixture file to testdata/mmm
- placed gzipped fixture in data/
- reworked tests to accomedate changes. less moving parts now.
- much faster import process with @transaction.commit_manually