add n_jobs to make_union

[amueller]

make_union currenlty has a comment saying "it would be nice to add a keyword-only n_jobs but that's not allowed in Python 2.x".

That's bogus as far as I can tell and we can easily add **kwargs there.

[jnothman]

I wonder whether that comment derived from a precursor when we were considering make_union(name=estimator)

[amueller]

maybe? Doesn't really matter now, though ;)