Implementation for assertSubset #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.
Oops. It seems phpunit functional api is not being tested... would be nice to run all tests against the functional API too...