Inspect, eql?, == and hash should work even when no attributes are specified

The following code does not cause the `inspect`, `eql?`, `==` and `hash` methods to be added:

``` ruby
class Foo
  extend Attributable
end
```

For now, the workaround is to call attributes with no arguments:

``` ruby
class Foo
  extend Attributable
  attributes
end
```

But note that this highlights a slight bug in `inspect`:

``` ruby
Foo.new.inspect => "<Foo >"
```
