New matcher: `have_attributes`

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

@myronmarston what about:
* ```an_object_with_attributes``` + single form * ```an_object_with_an_attribute```
*  ```with_attributes``` + single form * ```with_an_attribute```