The "Creative Commons
Attribution-NonCommercial-NoDerivs 3.0 Unported License" is an "open source" license. The source code itself is "open" and can be redistributed for free (beer not speech). But it is disappointing that they have opted to prohibit derivatives, and it makes their posting of the code to GitHub an interesting choice.
IANAL, but as I understand the license, yes it would violate the license. Although, if I released something under a no-derivatives license on github, I'd be less worried about pull requests and more worried about derivatives made outside GitHub.
No, the default state of software is copyrighted and not redistributable. You need a specific license to give you the right to modify and/or redistribute software (though I think copyright should be changed to allow private modification for certain purposes).
