Feature/Cassette#all_played

Hi 

This pull request about adding new helper function to Cassette to simplify (and make more readable) the testing that all request have been played from cassette. For example:

```
with vcr.use_cassette('cassette_with_one_request.yml') as cass:
        assert not cass.all_played
        requests.get('http://httpbin.org')
        assert cass.all_played
```