Direct modification of FK field now persists

[grigi]

Fixes #184

Done:

• Assigning to the FK field will correctly set the associated db-field
• Reading a nullalble FK field can now be None
• Nullalble FK fields reverse-FK is now also nullable
• Deleting a nullable FK field sets it to None

[coveralls]

Pull Request Test Coverage Report for Build 729

• 14 of 14 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 97.511%

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Totals
Change from base Build 718:	0.06%
Covered Lines:	2836
Relevant Lines:	2878

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 721

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 97.534%

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Totals
Change from base Build 718:	0.08%
Covered Lines:	2734
Relevant Lines:	2762

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💛 - Coveralls