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.

Agree, but just `attributes(...)` would but that's probably too generic, I have a minor preference for `an_object_with_attributes(..)` over `an_object_having_attributes(..)` but I'm ok with the latter for consistency.