through model expansion on m2m fields

[jerch]

PR for through model expansion on m2m fields. This is a quite big change of the m2m resolver logic, so it is likely to become a minor release.

TODO:

• optimize handler calls
• fix broken test in test_helpermodels.py
• adjust docs
• check cli commands
  • add through models to showdependencies

Also fixes #145.

[coveralls]

Pull Request Test Coverage Report for Build 16201408707

Details

• 46 of 46 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 95.086%

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Totals
Change from base Build 16200749067:	0.04%
Covered Lines:	1148
Relevant Lines:	1194

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💛 - Coveralls