return none from artifact_at

[rickwalder]

closes #48

[smallsco]

Hi @rickwalder ,

This is a great addition, unfortunately however it will break any customer code that assumes container.artifact_at() will always return an Artifact or raise a KeyError.

For that reason, I don't think we can accept this for the 1.x branch, which must maintain backward compatibility.

I would like to keep the PR open though, as incorporating this change makes sense for when we eventually publish a v2.0 of the library and can break BC at that point.