Initial implementation of playback middleware.

This allows any Rack-based app to play pre-recorded http://github.com/vcr/vcr responses for matching requests.

You can configure multiple cassettes (requests will fall through until handled):

```ruby
use CassetteDeck::Play, 'path/to/cassette'
use CassetteDeck::Play, 'path/to/another/cassette', match_requests_on: [:path]
```