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.
Oh, I get it. Basically it would be a switch for the comparison operator? Something like:

```php
function assertArrayPart($needle, $subject, $checkObjectIdentity = false, $message = null);
```
Default becomes `==` instead of `===`. It would be good to have this flexibility. I'll take a look at the comparator subframework too.