Allow ensuring a key does not exist in a dict

I am using the library to validate JSON responses in my REST API, by doing something like:

```
resp = requests.get(...)
assert Schema({...}) in resp.json()
```

(I have couple of utility functions in there to make this work with pytest.)

It would be nice to have a construct that allowed to describe a key that may NOT exist in the response. Consider example.

```
resp = request.patch(...remove-a-key...)
assert Schema({
Missing('that-key'): None
}) in resp.json()
```

...or maybe it could be in `Schema.__init__` as `exclude=['field1', 'field2']` or something.