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.
[IsEqual](https://github.com/sebastianbergmann/phpunit/blob/master/src/Framework/Constraint/IsEqual.php) uses the [comparator subframework](https://github.com/sebastianbergmann/comparator). It provides the foundation for all the equality checks in PHPUnit. It allows us to do delta comparisons with floats, to canonicalization xml, to compare recursive objects (this normally crashes PHP), and much more.