New matcher: `have_attributes`

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

> which we pick depends on if we favor consistency or more natural phrasing.

if *consistency* means doing:
```ruby
alias_matcher :an_object_ORIG_NAME, :ORIG_NAME
```

then by looking at other ```alias_matcher``` definitions, it seem like *natural phrasing* is mostly favored:

```ruby
alias_matcher :an_object_existing, :exist
alias_matcher :an_object_matching, :match
alias_matcher :an_object_responding_to, :respond_to
alias_matcher :an_object_satisfying, :satisfy
```

so I vote for ```an_object_with_attributes```