New matcher: `have_attributes`

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

I noticed in the build that the diff is odd:

https://travis-ci.org/rspec/rspec-expectations/jobs/27226588#L275

If you have an idea for how to fix that, feel free -- otherwise, my advice is to make it not diffable for now.