New matcher: `have_attributes`

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

The styles fine, you've just made a typo on the line above the first comment and inserted two spaces between the `to` and the `have`

```Ruby
it { is_expected.to have_attributes
it { is_expected.to  have_attributes
```