Implementation for assertArrayPart #1371

I probably will not have time to open this PR later this weekend so I decided to do it now, just in case the feature discussed at #1371 get's approved :D

In case it gets approved but with a different assertion name I'll be happy to update the PR, just ping me.

Cheers.
[TraversableContains](https://github.com/sebastianbergmann/phpunit/blob/master/src/Framework/Constraint/TraversableContains.php) allows using `==` or `===`. That way objects don't have to be the exact same instance, for example.