'denies' support

Hi,

it would be nice to add denies support to riot-gear, especially because with riot 0.12.5 we get:

```
exists is deprecated; please use denies.nil instead of asserts.exists
```

when we do something like...

``` ruby
asserts_json("id").exists
```

Instead, we should be able to do

``` ruby
denies_json("id").nil
```

What do you think? I can try to prepare a pull request for it.
