New matcher: `have_attributes`

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

i was simply suggesting possible aliases:
```ruby
expect(person).to an_object_with_attributes(:name => "Myron", :age => 32)
expect(person).to an_object_with_attribute(:name => "Myron)

expect(person).to  with_attributes(:name => "Myron", :age => 32)
expect(person).to with_an_attribute(:name => "Myron)
```