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.
@whatthejeff ok, I've just made a forced push with the fix so we don't pollute the commit history.