New matcher: `have_attributes`

see: https://github.com/rspec/rspec-expectations/issues/565

@myronmarston @JonRowe @cupakromer /cc

TODO's
- [ ] update README

I don't think `with_attributes` on its own makes sense.  Either `an_object_with_attributes` or `an_object_having_attributes` make sense to me -- which we pick depends on if we favor consistency or more natural phrasing.

@JonRowe, what do you think?