nullable properties

[serl]

This should address #73.

Anyway, we should first talk about this, as I'm not really sure of what I'm doing here.
The meat is in the first commit. Does it look good?
Then I updated the tests that broke other tests. I ended up setting the Json Schema test as non-strict (last commit). This makes all tests pass, but maybe we shouldn't have that disabled..? Also, this is going to break users' code...

[Korbeil]

Could you rebase onto master ? Just to avoid having old commit here.

[coveralls]

Pull Request Test Coverage Report for Build 392

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 86.289%

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Totals
Change from base Build 387:	0.03%
Covered Lines:	2983
Relevant Lines:	3457

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💛 - Coveralls

[serl]

Done!
(I'll take care about the squashing at the end)

[Korbeil]

Could you squash pls ?
And if you have time to make same patch for 4.x branch that would be awesome (if not I can handle that 😉)